Point to the book where possible, and use hyperlinks to github to refer to files not included in the book.

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Add developer documentation to explain room DAG concepts like `outliers` and `state_groups` (#10464)

This is essentially an implementation of the proposal made at https://hackmd.io/@richvdh/BJYXQMQHO, though the details have ended up looking slightly different.