Add search bar to landing page

What does this MR do and why?

This MR adds the search bar section to landing page in Hugo Docs site.

Screenshots, screen recordings, or links to review app

Wide screen size:

Mobile screen size:

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-group/gitlab-docs-hugo/-/blob/main/doc/setup.md.
2. Go to landing page: http://localhost:1313/ and make sure search is working (search history is available). Check different screen sizes to make sure it matches the prod. site landing page.

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.