Runtime APIs for node-side code (#1401)

* create a README on Runtime APIs

* add ParaId type

* write up runtime APIs

* more preamble

* rename

* rejig runtime APIs

* add occupied_since to `BlockNumber`

* improve group_for_core

* improve docs on availability cores runtime API

* guide: freed -> free

* Update roadmap/implementers-guide/src/runtime-api/README.md

Co-authored-by: Peter Goodspeed-Niklaus <[email protected]>

* add explicit block parameter to runtime API fns

Co-authored-by: Peter Goodspeed-Niklaus <[email protected]>