Docs: Clarify how Geo sites can be used

Follow up #12280 (comment 1009401714):

1. Make it clear early in the Geo docs that:

Secondary Geo sites transparently behave like the primary, by default. Therefore Geo sites can be load balanced, with the caveat that you must use sticky sessions to avoid authentication failures and cross-site request errors.

1. Search the docs for read-only and adjust for the fact that secondary proxying is enabled by default
2. Remove https://docs.gitlab.com/ee/user/admin_area/geo_sites.html#multiple-secondary-sites-behind-a-load-balancer since it is now superseded by https://docs.gitlab.com/ee/administration/geo/secondary_proxy/location_aware_external_url.html