* create a README on Runtime APIs
* add ParaId type
* write up runtime APIs
* more preamble
* rename
* rejig runtime APIs
* add occupied_since to `BlockNumber`
* skeleton crate for runtime API subsystem
* improve group_for_core
* improve docs on availability cores runtime API
* guide: freed -> free
* add primitives for runtime APIs
* create a v1 ParachainHost API trait
* guide: make validation code return `Option`al.
* skeleton runtime API helpers
* make parachain-host runtime-generic
* skeleton for most runtime API implementation functions
* guide: add runtime API helper methods
* implement new helpers of the inclusion module
* guide: remove retries check, as it is unneeded
* implement helpers for scheduler module for Runtime APIs
* clean up `validator_groups` implementation
* implement next_rotation_at and last_rotation_at
* guide: more helpers on GroupRotationInfo
* almost finish implementing runtime APIs
* add explicit block parameter to runtime API fns
* guide: generalize number parameter
* guide: add group_responsible to occupied-core
* update primitives due to guide changes
* finishing touches on runtime API implementation; squash warnings
* break out runtime API impl to separate file
* add tests for next_up logic
* test group rotation info
* point to filed TODO
* remove unused TODO [now]
* indentation
* guide: para -> para_id
* rename para field to para_id for core meta
* remove reference to outdated AvailabilityCores type
* add an event in `inclusion` for candidates being included or timing out
* guide: candidate events
* guide: adjust language
* Candidate events type from guide and adjust inclusion event
* implement `candidate_events` runtime API
* fix runtime test compilation
* max -> min
* fix typos
* guide: add `RuntimeAPIRequest::CandidateEvents`
Robert Habermeier