Google search UX improvements

What does this MR do and why?

UX improvements based on feedback in #1377 (closed):

Homepage

• Removes the "Explore all topics" links from the homepage search box. This link is not related to searching, and despite its prominence only gets ~5% of clicks on the homepage.

Search results page (/search)

Visual:

• Search form on the /search page is now the same width and style as the homepage and interior page instances.
• Fixes a stacking issue on the search form text input (input box appeared over the navbar when focused).
• New filter labels to better fit search context. Functionally, filters still map to site navigation sections.
• Fixes pager styling (link colors were purple; should be dark grey).

Functional:

• Fixes a bug where "no results" text could show prior to form submission (but after starting to type a query) on the /search page
• Changes result summary ("Showing x of y results") to max out at 100 total results. While Google usually finds more results than this, we can only fetch the first 100 from the API, so this the max a user can browse.

Screenshots, screen recordings, or links to review app

Review app: https://gps-search-ux-improvements.docs.gitlab-review.app/index.html

Before	After

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.