Draft: Build centralized documentation of GitLab feature flags to aid discovery

What does this MR do?

This MR adds scripts to:

1. Adds a script, collect-feature-flags, which:
   1. Scans the entire Ruby codebase, using Rubocop and a custom cop, for calls to Feature.enabled?
   2. Writes out a database of feature flag uses to doc/administration/feature_flags/uses.yml
2. Adds a file, doc/administration/feature_flags/help.yml for documenting feature flags
3. Adds a script, document-feature-flags, which:
   1. Synthesizes feature flag documentation (help.yml) and uses (uses.yml)
   2. Writes out the synthesized information to a human-readable document, doc/administration/feature_flags/directory.md

Example

Feature	Flag	Default	GitLab.com	Reason	Uses
Go proxy for GitLab	go_proxy	disabled	disabled	Performance ...	lib/api/go_proxy.rb:48:27
	instance_statistics	enabled			app/controllers/admin/instance_statistics_controller.rb:16:23
	serverless_domain	disabled			app/controllers/admin/serverless/domains_controller.rb:68:23
	soft_email_confirmation	disabled			app/controllers/concerns/confirm_email_warning.rb:13:38 app/controllers/confirmations_controller.rb:16:43 app/controllers/registrations/welcome_controller.rb:53:23 app/controllers/registrations_controller.rb:98:5

Related issues

Closes #290719

Author's checklist (required)

• Follow the Documentation Guidelines and Style Guide.
• If you have Developer permissions or higher:
  • Ensure that the product tier badge is added to doc's h1.
  • Apply the documentation label, plus:
    • The corresponding DevOps stage and group labels, if applicable.
    • development guidelines when changing docs under doc/development/*, CONTRIBUTING.md, or README.md.
    • development guidelines and Documentation guidelines when changing docs under development/documentation/*.
    • development guidelines and Description templates (.gitlab/*) when creating/updating issue and MR description templates.
  • Assign the designated Technical Writer.

Do not add the feature, frontend, backend, ~"bug", or database labels if you are only updating documentation. These labels will cause the MR to be added to code verification QA issues.

When applicable:

• Update the permissions table.
• Link docs to and from the higher-level index page, plus other related docs where helpful.
• Add the product tier badge accordingly.
• Add GitLab's version history note(s).
• Add/update the feature flag section.

Review checklist

All reviewers can help ensure accuracy, clarity, completeness, and adherence to the Documentation Guidelines and Style Guide.

1. Primary Reviewer

• Review by a code reviewer or other selected colleague to confirm accuracy, clarity, and completeness. This can be skipped for minor fixes without substantive content changes.

2. Technical Writer

• Technical writer review. If not requested for this MR, must be scheduled post-merge. To request for this MR, assign the writer listed for the applicable DevOps stage.
  • Ensure docs metadata are present and up-to-date.
  • Ensure Technical Writing and documentation are added.
  • Add the corresponding docs:: scoped label.
  • If working on UI text, add the corresponding UI Text scoped label.
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  • Add twfinished if Technical Writing team work on the MR is complete but it remains open.

For more information about labels, see Technical Writing workflows - Labels.

For suggestions that you are confident don't need to be reviewed, change them locally and push a commit directly to save others from unneeded reviews. For example:

• Clear typos, like this is a typpo.
• Minor issues, like single quotes instead of double quotes, Oxford commas, and periods.

For more information, see our documentation on Merging a merge request.

3. Maintainer

1. Review by assigned maintainer, who can always request/require the above reviews. Maintainer's review can occur before or after a technical writer review.
2. Ensure a release milestone is set.
3. If there has not been a technical writer review, create an issue for one using the Doc Review template.