Add search filtering options

What does this MR do and why?

Adds a set of checkboxes for filtering search results by site section.

Closes #1311 (closed)

Screenshots, screen recordings, or links to review app

Before	After

How to set up and validate locally

This can be tested on the review app: https://1311-gps-filters.docs.gitlab-review.app/

Or, to test locally:

1. Configure a local GitLab Docs environment: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/setup.md.
2. Compile the site with Google search: make clean && GOOGLE_SEARCH_KEY="AIzaSyAUCkVNs7JsHYKISjVrVv1PGXg9WQr1MCQ" SEARCH_BACKEND="google" bundle exec nanoc compile
3. Run some searches and try filtering results. Results should be pages within the selected section(s).

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.