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)

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