Add a section for Solutions docs

What does this MR do and why?

Adds elements to distinguish Solutions Architecture content:

• New tier-style badge for pages under /ee/solutions
• New search filter option

Related to gitlab#430270 (closed)

Screenshots, screen recordings, or links to review app

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 for Google search: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/doc/search.md?ref_type=heads#local-build-with-google-programmable-search
3. There is not any Solutions Architecture content yet, so create a test page here gitlab/doc/solutions/index.md that contains at least a title tag (h1)
4. Compile and view the site: make view
5. Run a search and verify there's a new checkbox for Solutions docs
6. View the test page at http://localhost:3000/ee/solutions/ and verify the Solutions badge appears alongside the title.

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.