Add 3PID API flow diagrams

The [3PID API flow diagrams](https://gist.github.com/jryans/839a09bf0c5a70e2f36ed990d50ed928) have been quite helpful when working on the recent privacy project.

I think it would be useful for the community if we store them somewhere more official. Since they describe multiple services working together, they don't really fit in a single API document. Maybe a new page for this purpose and the related API methods can link to the flow diagrams?

I am sure they will need some tweaks for official use, so feel free to pass along that feedback as well.

Start with the linked 3PID API flow diagrams and inspect the related API method documentation in the Matrix specification. Create an official page for cross-service flows, incorporate any needed tweaks, and link it from the related API methods.

Written by the indexing model from the issue text.