PRDoc new schema (#1946)

## Overview

This PR brings in the new version of prdoc v0.0.6 and allows:
- local schema
- local config
- local template

It also fixes the existing prdoc files to match the new schema.

## todo

- [x] add a brief doc/tldr to help contributors get started
- [x] test CI
- [x] finalize schema
- [x] publish the next `prdoc` cli version (v0.0.7 or above)

---------

Co-authored-by: Egor_P <egor@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

docs/DOCUMENTATION_GUIDELINES.md

@@ -225,7 +225,7 @@ For the top-level pallet docs, consider the following template:
 //!
 //! ## Pallet API
 //!
-//! <Reminder: inside the [`pallet`] module, a template that leads the reader to the relevant items is auto-generated. There is no need to repeat 
+//! <Reminder: inside the [`pallet`] module, a template that leads the reader to the relevant items is auto-generated. There is no need to repeat
 //! things like "See Config trait for ...", which are generated inside [`pallet`] here anyways. You can use the line below as-is:>
 //!
 //! See the [`pallet`] module for more information about the interfaces this pallet exposes, including its
@@ -349,3 +349,7 @@ Consider the fact that, similar to dispatchables, these docs will be part of the
 and might be used by wallets and explorers.
 
 Specifically for `error`, explain why the error has happened, and what can be done in order to avoid it.
+
+## Documenting Changes/PR
+
+See [PRDoc](./prdoc.md).