* Add a `last change` footer to the implementers guide
Some of the newcomers were noticing outdated pages in the implementer's guide.
This idea came up as a heuristic for how up-to-date an individual page is.
* Update `build-implementers-guide` CI job
* start graphing message flow between subsystems
This commit includes messages originating from the first three subsystems
(alphabetically).
* use standard arrowhead names
* add messages from bitfield distribution && use circo layout
* finish adding all subsystem message traffic to graph
* reduce line spam
* start work on graphing v1 types
* finish graphing candidate-related structs
* show hash relations, format
* show provenance of ValidityAttestation
* add the rest of the v1 polkadot primitives
* add type diagram from polkadot::parachain::primitives
* start work digramming the path to backing
* diagram message flow from collation generation -> candidate selection
* document flow through CandidateBacking, CandidateValidation
* graph data flow through StatementDistribution
* trace data flow through PoVDistribution to Backing
* finish documenting node side subsystem data flow
Marcin S