Refer to primary and secondary nodes consistently in Geo documentation

Re: Comment https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8117#note_113357512

• In general, in documentation, refer to Geo nodes as **primary** node and **secondary** node, without a Geo prefix.

@gl-docsteam Please add any related consistency items to this (or close if redundant with another issue I did not find).