Add a script to flag expired content

What does this MR do and why?

This MR adds two new CLI commands to accomplish the "find expired content" maintenance task:

1. A generic search method (make search-content query="whatever") to find a given string in any of the docs projects.
2. The more targeted make find-expired-content script to specifically find expired content that needs to be removed.

Expired content is marked in docs using comments like this:

<!--- start_remove The following content will be removed on remove_date: 'YYYY-MM-DD' -->

Cleaning these up monthly is part of the monthly tasks rotation (which we can start up again after we get this last one merged, and the template updated!).

Related:

• https://docs.gitlab.com/development/documentation/styleguide/deprecations_and_removals/
• #270 (closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/technical-writing/docs-gitlab-com/-/blob/main/doc/setup.md.
2. Try the search command:

make search-content query="{{< tabs >}}"

3. Try running the new maintenance script: make find-expired-content. Expected results: no expired content found.
4. Create some "expired content" by editing a few pages in ../gitlab/doc and adding expiration tags, like:

A date in the past, that should be returned by the script:

<!--- start_remove The following content will be removed on remove_date: '2020-12-31' -->

A date in the future, which should not:

<!--- start_remove The following content will be removed on remove_date: '2025-11-15' -->

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.