Fix broken internal links in docs

doc/install/installation.md

@@ -6,7 +6,7 @@ type: howto
 
 This is the official installation guide to set up a production GitLab server
 using the source files. To set up a **development installation** or for many
-other installation options, see the [main installation page](index.md).
+other installation options, see the [main installation page](README.md).
 It was created for and tested on **Debian/Ubuntu** operating systems.
 Read [requirements.md](requirements.md) for hardware and operating system requirements.
 If you want to install on RHEL/CentOS, we recommend using the

doc/user/clusters/applications.md

@@ -95,7 +95,7 @@ CI/CD](../../ci/README.md), the open-source continuous integration
 service included with GitLab that coordinates the jobs. When installing
 the GitLab Runner via the applications, it will run in **privileged
 mode** by default. Make sure you read the [security
-implications](../project/clusters/index.md/#security-implications) before doing so.
+implications](../project/clusters/index.md#security-implications) before doing so.
 
 NOTE: **Note:**
 The

doc/user/project/repository/branches/index.md

@@ -13,7 +13,7 @@ After pushing your changes to a new branch, you can:
 
 - Create a [merge request](../../merge_requests/index.md)
 - Perform inline code review
-- [Discuss](../../discussions/index.md) your implementation with your team
+- [Discuss](../../../discussions/index.md) your implementation with your team
 - Preview changes submitted to a new branch with [Review Apps](../../../../ci/review_apps/index.md).
 
 With [GitLab Starter](https://about.gitlab.com/pricing/), you can also request