Docs: add index for /customization/*

What does this MR do?

• Adds index for /customization/
• Deprecates a redundant document (customization/welcome_message.md)
• Redirects deprecated doc to customization/branded_login_page.md
• Update screenshot

Related issues

• For https://gitlab.com/gitlab-org/gitlab-ce/issues/55234

Moving docs to a new location?

• Make sure the old link is not removed and has its contents replaced with a link to the new location.
• Make sure internal links pointing to the document in question are not broken.
• Search and replace any links referring to old docs in GitLab Rails app, specifically under the app/views/ and ee/app/views (for GitLab EE) directories.
• Make sure to add redirect_from to the new document if there are any Disqus comments on the old document thread.
• Update the link in features.yml (if applicable)
• If working on CE and the ee-compat-check jobs fails, submit an MR to EE with the changes as well: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9578
• Ping one of the technical writers for review
• Merge EE (https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/9578) MR first, this MR afterward.
• Update the global nav with the new index => https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/413