Rename README.md files to index.md

Now that we have server-side redirects, we can think about renaming all the README.md files to index.md, with a server-side redirect for each as a backup. Then we can remove the README.md test in lint-doc.sh: https://gitlab.com/gitlab-org/gitlab/-/blob/master/scripts/lint-doc.sh#L42-56

Related to gitlab#30718 (closed).

Bulk change steps:

• Review/merge !1922 (merged)
• Review/merge (in any order):
  • gitlab!64895 (merged)
  • gitlab!64885 (merged)
  • gitlab!64886 (merged)
  • gitlab!64887 (merged)
  • gitlab!64893 (merged)
  • gitlab-runner!2990 (merged)
  • omnibus-gitlab!5409 (merged)

Steps See above for bulk process

1. Create a merge request in one of the projects that have a README.md, and redirect them to index.md (example gitlab!53328 (merged)).
2. Create a merge request in gitlab-docs and edit content/_data/readmes.yaml to remove the files you redirected in the previous step.
3. Merge the redirect MR.
4. Merge the gitlab-docs MR.
5. Check the redirected items in the list below.

• runner/development/README.html
• runner/monitoring/README.html
• runner/executors/README.html
• runner/commands/README.html
• runner/faq/README.html
• runner/release_process/README.html
• omnibus/development/openshift/README.html
• omnibus/development/README.html
• omnibus/docker/README.html
• omnibus/gitlab-mattermost/README.html
• omnibus/package-information/README.html
• omnibus/release/README.html
• omnibus/settings/README.html
• omnibus/common_installation_problems/README.html
• omnibus/gitlab-ci/README.html
• omnibus/build/README.html
• omnibus/update/README.html
• omnibus/maintenance/README.html
• omnibus/roles/README.html
• omnibus/architecture/registry/README.html
• omnibus/architecture/README.html
• omnibus/README.html
• ee/downgrade_ee_to_ce/README.html
• ee/security/README.html
• ee/university/README.html
• ee/intro/README.html
• ee/development/README.html
• ee/api/README.html
• ee/integration/README.html
• ee/administration/auth/README.html
• ee/legal/README.html
• ee/install/README.html
• ee/ci/chatops/README.html
• ee/ci/examples/deployment/README.html
• ee/ci/examples/README.html
• ee/ci/docker/README.html
• ee/ci/services/README.html
• ee/ci/yaml/README.html
• ee/ci/runners/README.html
• ee/ci/triggers/README.html
• ee/ci/quick_start/README.html
• ee/ci/ssh_keys/README.html
• ee/ci/variables/README.html
• ee/ci/README.html
• ee/raketasks/README.html
• ee/migrate_ci_to_ce/README.html
• ee/update/README.html
• ee/gitlab-basics/README.html
• ee/README.html
• ee/ssh/README.html