Convert guide from single markdown file to mdbook (#1247)

* move old implementers' guide, add skeleton of new

* Split the old implementers' guide into the new one's sections

This is mostly a straightforward copying operation, moving the
appropriate sections from the old guide to the new. However, there
are certain differences between the old text and the new:

- removed horizontal rules between the sections
- promoted headers appropriately within each section
- deleted certain sections which were in the old guide's ToC but
  which were not actually present in the old guide.
- added Peer Set Manager to the new ToC

* remove description headers

It is redundant and unnecessary. Descriptions fall directly under the
top-level header for any given section.

* add stub description of the backing module

* add stub description for the availability module

* add stub description for collators

* add stub description for validity

* add stub description for utility

* highlight TODO and REVIEW comments

* add guide readme describing how to use mdbook

* fix markdownlint lints

* re-title parachains overview

* internal linking for types

* module and subsystem internal links

* .gitignore should have a trailing newline

* node does not have modules, just subsystems

polkadot/roadmap/implementors-guide/src/node/utility/provisioner.md

@@ -0,0 +1,30 @@
+# Provisioner
+
+This subsystem is responsible for providing data to an external block authorship service beyond the scope of the [Overseer](/node/overseer.html) so that the block authorship service can author blocks containing data produced by various subsystems.
+
+In particular, the data to provide:
+
+- backable candidates and their backings
+- signed bitfields
+- misbehavior reports
+- dispute inherent
+    > TODO: needs fleshing out in validity module, related to blacklisting
+
+## Protocol
+
+Input:
+
+- Bitfield(relay_parent, signed_bitfield)
+- BackableCandidate(relay_parent, candidate_receipt, backing)
+- RequestBlockAuthorshipData(relay_parent, response_channel)
+
+## Functionality
+
+Use `StartWork` and `StopWork` to manage a set of jobs for relay-parents we might be building upon.
+Forward all messages to corresponding job, if any.
+
+## Block Authorship Provisioning Job
+
+Track all signed bitfields, all backable candidates received. Provide them to the `RequestBlockAuthorshipData` requester via the `response_channel`. If more than one backable candidate exists for a given `Para`, provide the first one received.
+
+> TODO: better candidate-choice rules.