Add geo sites page to nav bar, deprecate geo nodes

What does this MR do and why?

Accompanying MR gitlab!116438 (merged)

gitlab#369140 is adding GeoSites API endpoints to supersede GeoNodes on API v5

• Add a link to new GeoSites API endpoint that's introduced in gitlab!115654 (merged)
• Deprecate GeoNodes API endpoint

Because GeoNodes endpoint is getting deprecated, not removed, introduce a new GeoSites page, but mark GeoNodes as deprecated. More discussed in the comment here gitlab#369140 (comment 1315952358)

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