Arvados

The Architecture page at https://doc.arvados.org/v2.1/architecture/federation.html and the admin section at https://doc.arvados.org/v2.1/admin/federation.html have considerable overlap, but neither clearly lays out the types of federation and what some example use cases are.

We should upgrade the Architecture page to an entire Federation section, with these pages:

• Overview
• Peer federation
• Login federation
• public-data access federation (need a new term for this type!)
• Federation mechanics (move the "federation and signatures" section from https://doc.arvados.org/v2.1/architecture/manifest-format.html#federationsignatures here; also this is where some of the text from https://doc.arvados.org/v2.1/architecture/federation.html will go)

For each federation type the page needs to clarify:

• when this type of federation is appropriate
• how to configure the clusters involved
• what the functional impact is (e.g. multisite search works in this particular way, login works like this, what happens when a user on cluster a requests a uuid that lives on another cluster, what happens when a workflow does that, etc)
• security considerations

Needless to say, it's fine to say "not yet implemented" for stuff that does not work.