Geo: Update Geo documentation to include the concept of a site

At https://docs.gitlab.com/ee/development/geo.html#glossary (and all through our documentation) we refer to the Primary node and Secondary node(s). This is fine, but for multi-node Geo setups, I feel when we're talking about all nodes within a Primary or Secondary configuration we need a new term for this. Expanding upon the Glossary, I'd like to add a new item called 'site'.

This is what we have now (in table form):

Definition	Description
Primary node	A primary node is the single node in a Geo setup that read-write capabilities. It’s the single source of truth and the Geo secondary nodes replicate their data from there. In a Geo setup, there can only be one primary node. All secondary nodes connect to that primary.
Secondary node	A secondary node is a read-only replica of the primary node running in a different geographical location.

And with the addition of Geo site definitions (obviously the description section needs work):

Definition	Description
Primary site	Contains all primary nodes and has read-write capabilities. It is the single source of truth where Geo secondary sites replicate their data from. In a Geo setup, there can only be one primary site.
Primary node	A primary node is a single node within the primary site.
Secondary site	Contains all secondary nodes but does not possess read-write capabilities. It is a read-only replica of the primary site running in potentially different geographical locations.
Secondary node	A secondary node is a single node within the secondary site.

This also brings greater consistency to Geo when in the context of DR as DR locations are often referred to as 'sites'.

Thoughts @geo-team ?