Docs: Geo: Mention read-only API use-case for Geo

Problem to solve

Follow up for !17016 (comment 218952645):

Example use case: You have lots of runners that do lots of read-only API calls. You have them make those calls to read-only.example.com, which uses the closest Geo secondary. Actually, now that I think about it, this use case could include the primary as well, with no ill consequence, as long as the caller does not try to write anything. If customers find value in this, we should make this use-case known as well.

This may or may not be a good idea, and is not necessarily proven.

Further details

Proposal

If the use-case makes sense, the implementation is basically the same as setting up a geolocation-aware Git URL, so we could potentially add a small description somewhere and link to that How-to. And modify the How-to to mention this use-case as well.

Who can address the issue

Geo team

Other links/references

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