Add a search filter for tutorials

What does this MR do and why?

Adds a search filter option for Tutorials. Previously tutorials were included in "User docs."

The Tutorials filter should include pages that meet either of these conditions:

A) Located under "Tutorials" in the global navigation B) Have a title that starts with Tutorial:

Closes #1713 (closed)

Screenshots, screen recordings, or links to review app

Review app examples:

• https://1713-tutorials-filter-gps.docs.gitlab-review.app/search/?q=git&page=1&filters=tutorials
• https://1713-tutorials-filter-gps.docs.gitlab-review.app/search/?q=CI&page=1&filters=tutorials

Before	After

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/gitlab-docs/-/blob/main/doc/setup.md.
2. Compile the site to use Google search: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/search.md?ref_type=heads#local-build-with-google-programmable-search
3. Run a search and visit the results page
4. Verify you can filter the results to only include Tutorials pages
5. Verify the URL includes &filters=tutorials when the Tutorials checkbox is checked

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.

Closes #1713 (closed)