Move the Omnibus update doc to the GitLab repo

This is mostly moving the content of
https://gitlab.com/gitlab-org/omnibus-gitlab/-/blob/f87042ca218a1f834e17dc769da5eb5f7356aac2/doc/update/index.md
in the GitLab repository, with some minor changes to headings.

doc/update/package/convert_to_ee.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Convert Community Edition to Enterprise Edition **(FREE SELF)**
+
+To convert an existing GitLab Community Edition (CE) server installed using the Omnibus GitLab
+packages to GitLab [Enterprise Edition](https://about.gitlab.com/pricing/) (EE), you install the EE
+package on top of CE.
+
+Converting from the same version of CE to EE is not explicitly necessary, and any standard upgrade
+(for example, CE 12.0 to EE 12.1) should work. However, in the following steps we assume that
+you are upgrading the same version (for example, CE 12.1 to EE 12.1), which is **recommended**.
+
+WARNING:
+When updating to EE from CE, avoid reverting back to CE if you plan on going to EE again in the
+future. Reverting back to CE can cause
+[database issues](index.md#500-error-when-accessing-project--settings--repository)
+that may require Support intervention.
+
+The steps can be summed up to:
+
+1. Find the currently installed GitLab version:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   sudo apt-cache policy gitlab-ce | grep Installed
+   ```
+
+   The output should be similar to: `Installed: 13.0.4-ce.0`. In that case,
+   the equivalent Enterprise Edition version will be: `13.0.4-ee.0`. Write this
+   value down.
+
+   **For CentOS/RHEL**
+
+   ```shell
+   sudo rpm -q gitlab-ce
+   ```
+
+   The output should be similar to: `gitlab-ce-13.0.4-ce.0.el8.x86_64`. In that
+   case, the equivalent Enterprise Edition version will be:
+   `gitlab-ee-13.0.4-ee.0.el8.x86_64`. Write this value down.
+
+1. Add the `gitlab-ee` [Apt or Yum repository](https://packages.gitlab.com/gitlab/gitlab-ee/install):
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   curl --silent "https://packages.gitlab.com/install/repositories/gitlab/gitlab-ee/script.deb.sh" | sudo bash
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   curl --silent "https://packages.gitlab.com/install/repositories/gitlab/gitlab-ee/script.rpm.sh" | sudo bash
+   ```
+
+   The above command will find your OS version and automatically set up the
+   repository. If you are not comfortable installing the repository through a
+   piped script, you can first
+   [check its contents](https://packages.gitlab.com/gitlab/gitlab-ee/install).
+
+1. Next, install the `gitlab-ee` package. Note that this will automatically
+   uninstall the `gitlab-ce` package on your GitLab server. `reconfigure`
+   Omnibus right after the `gitlab-ee` package is installed. **Make sure that you
+   install the exact same GitLab version**:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   ## Make sure the repositories are up-to-date
+   sudo apt-get update
+
+   ## Install the package using the version you wrote down from step 1
+   sudo apt-get install gitlab-ee=13.0.4-ee.0
+
+   ## Reconfigure GitLab
+   sudo gitlab-ctl reconfigure
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   ## Install the package using the version you wrote down from step 1
+   sudo yum install gitlab-ee-13.0.4-ee.0.el8.x86_64
+
+   ## Reconfigure GitLab
+   sudo gitlab-ctl reconfigure
+   ```
+
+1. Now go to the GitLab admin panel of your server (`/admin/license/new`) and
+   upload your license file.
+
+1. After you confirm that GitLab is working as expected, you may remove the old
+   Community Edition repository:
+
+   **For Debian/Ubuntu**
+
+   ```shell
+   sudo rm /etc/apt/sources.list.d/gitlab_gitlab-ce.list
+   ```
+
+   **For CentOS/RHEL**
+
+   ```shell
+   sudo rm /etc/yum.repos.d/gitlab_gitlab-ce.repo
+   ```
+
+That's it! You can now use GitLab Enterprise Edition! To update to a newer
+version, follow [Update using the official repositories](index.md#upgrade-using-the-official-repositories).
+
+NOTE:
+If you want to use `dpkg`/`rpm` instead of `apt-get`/`yum`, go through the first
+step to find the current GitLab version and then follow
+[Update using a manually-downloaded package](index.md#upgrade-using-a-manually-downloaded-package).

doc/update/package/downgrade.md

+---
+stage: Enablement
+group: Distribution
+info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#designated-technical-writers
+---
+
+# Downgrade **(FREE SELF)**
+
+This section contains general information on how to revert to an earlier version
+of a package.
+
+WARNING:
+You must at least have a database backup created under the version you are
+downgrading to. Ideally, you should have a
+[full backup archive](../../raketasks/backup_restore.md#back-up-gitlab)
+on hand.
+
+The example below demonstrates the downgrade procedure when downgrading between minor
+and patch versions (for example, from 13.0.6 to 13.0.5).
+
+When downgrading between major versions, take into account the
+[specific version changes](index.md#version-specific-changes) that occurred when you upgraded
+to the major version you are downgrading from.
+
+These steps consist of:
+
+- Stopping GitLab
+- Removing the current package
+- Installing the old package
+- Reconfiguring GitLab
+- Restoring the backup
+- Starting GitLab
+
+Steps:
+
+1. Stop GitLab and remove the current package:
+
+   ```shell
+   # If running Puma
+   sudo gitlab-ctl stop puma
+
+   # Stop sidekiq
+   sudo gitlab-ctl stop sidekiq
+
+   # If on Ubuntu: remove the current package
+   sudo dpkg -r gitlab-ee
+
+   # If on Centos: remove the current package
+   sudo yum remove gitlab-ee
+   ```
+
+1. Identify the GitLab version you want to downgrade to:
+
+   ```shell
+   # (Replace with gitlab-ce if you have GitLab FOSS installed)
+
+   # Ubuntu
+   sudo apt-cache madison gitlab-ee
+
+   # CentOS:
+   sudo yum --showduplicates list gitlab-ee
+   ```
+
+1. Downgrade GitLab to the desired version (for example, to GitLab 13.0.5):
+
+   ```shell
+   # (Replace with gitlab-ce if you have GitLab FOSS installed)
+
+   # Ubuntu
+   sudo apt install gitlab-ee=13.0.5-ee.0
+
+   # CentOS:
+   sudo yum install gitlab-ee-13.0.5-ee.0.el8
+   ```
+
+1. Reconfigure GitLab:
+
+   ```shell
+   sudo gitlab-ctl reconfigure
+   ```
+
+1. [Restore GitLab](../../raketasks/backup_restore.md#restore-for-omnibus-gitlab-installations)
+   to complete the downgrade.