Merge branch 'docs-add-index-customization' into 'master'

Docs: add index for /customization/*

See merge request gitlab-org/gitlab-ce!25298

doc/customization/help_message.md

+# GitLab Help custom text
+
+In larger organizations it is useful to have information about who has the responsibility of maintaining the company GitLab server.
+
+1. Navigate to the admin area, click on **Preferences** and expand **Help page**.
+
+1. Under **Help text** fill in the required information about the person(s) administering GitLab or any other information relevant to your needs.
+
+    ![help message](help_message/help_text.png)
+
+1. After saving the page this information will be shown on the GitLab login page and on the GitLab `/help` page (e.g., <https://gitlab.com/help>).
+
+    ![help text on help page](help_message/help_text_on_help_page.png)

doc/customization/index.md

+---
+description: Learn how to customize GitLab's appearance for self-managed installations.
+---
+
+# Customizing GitLab's appearance **[CORE ONLY]**
+
+For GitLab self-managed instances, it's possible to customize
+a few pages.
+
+Read through the following documents to adjust GitLab's
+look and feel to meet your needs:
+
+- [Custom login page](branded_login_page.md)
+- [Custom header and email logo](branded_page_and_email_header.md)
+- [Custom favicon](favicon.md)
+- [Libravatar](libravatar.md)
+- [New project page](new_project_page.md)
+- [Custom `/help` message](help_message.md)
\ No newline at end of file

doc/customization/welcome_message.md

-# Customize the complete sign-in page
-
-Please see [Branded login page](branded_login_page.md)
-
-# Add a welcome message to the sign-in page (GitLab Community Edition)
-
-It is possible to add a markdown-formatted welcome message to your GitLab
-sign-in page. Users of GitLab Enterprise Edition should use the [branded login
-page feature](branded_login_page.md) instead.
-
-The welcome message (extra_sign_in_text) can now be set/changed in the Admin UI.
-Admin area > Settings
+This document was moved to [another location](branded_login_page.md).