Add geo_sites API endpoint

What does this MR do and why?

Add geo_sites API endpoint that will supersede geo_nodes

This MR is a part of the issue Geo: New API endpoint for geo_sites to supersede geo_nodes

The issue is split into several MRs that should be merged in sequential order:

1. Add GeoSite and GeoSiteStatus grape entities — Merged ✅
2. Add geo sites fixtures schemas — Merged ✅
3. This MR: Add geo_sites API endpoint
4. Add Geo Sites API documentation
5. Add deprecation warning to Geo Nodes API doc
6. Add deprecation notice for geo_nodes API endpoints
7. Add geo_sites documentation to the nav bar

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR 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 MR.

Related to #369140