Geo docs: Be consistent, say "primary site", not "primary"

Proposal

• "Site" refers to an entire GitLab Geo installation (which may consist of multiple nodes)
• "Node" refers to an individual server/machine within that site

Good

• primary site
• secondary site
• secondary sites

Bad

• primary site
• secondary site
• secondary sites
• primary node
• secondary node
• secondary nodes
• primary
• secondary
• secondaries

We need to update a number of references.