Add deprecation warning to Geo Nodes API doc

What does this MR do and why?

This MR adds a deprecation warning to the Geo Nodes API doc because it would be deprecated in %16.0 and superseded by Geo Sites API.

This MR references a new Geo Sites API page from !115654 (merged), so it should be merged only after geo sites documentation is merged

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. Add geo_sites API endpoint
4. Add Geo Sites API documentation
5. This MR: 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

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Related to #369140