Various Guide Improvements (#1270)

* types chapter & candidate types

* Factor out some message types

* backing and availability types.

* spacing

* finish thought

* info on signed payloads

* update links

* explicit wrapper types

* add remaining message types

* add runtime API subsystem

* adjust language

* tweak candidate validation message

* find homes for some misfit types

* embed abridged within full candidate receipt

* Update roadmap/implementors-guide/src/SUMMARY.md

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* Update roadmap/implementors-guide/src/SUMMARY.md

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>

* adjust text on candidate selection message to match

Co-authored-by: Peter Goodspeed-Niklaus <coriolinus@users.noreply.github.com>