Fixes https://github.com/paritytech/polkadot-sdk/issues/5647

- [x]  create new command (reusing original py module)
- [x]  add unit-test cases (just the fact of proxy)
- [x]  update docs

Prdoc is now published as a Parity fork under the
[`parity-prdoc`](https://crates.io/crates/parity-prdoc) crate after the
directions diverged from the ideas of the original creator (discussions
[here](https://github.com/paritytech/prdoc/pull/40) and
[here](https://github.com/paritytech/prdoc/issues/36)).
Now updating the install instructions here.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

Updating the prdoc doc file to be a bit more useful for new contributors
and adding a SemVer section.

---------

Signed-off-by: Oliver Tale-Yazdi <oliver.tale-yazdi@parity.io>

This PR is a continuation of
https://github.com/paritytech/polkadot-sdk/pull/2102 and part of an
initiative started here https://hackmd.io/@romanp/rJ318ZCEp
What has been done:
- The content under `docs/*` (with the exception of `docs/mermaid`) has
been moved to `docs/contributor/`
- Developer Hub has been renamed to Polkadot SDK Docs, and the crate has
been renamed from `developer-hub` to `polkadot-sdk-docs`
- The content under `developer-hub/*` has been moved to `docs/sdk`

---
Original PR https://github.com/paritytech/polkadot-sdk/pull/2565, it has
been close due to too many rebase conflicts

---------

Co-authored-by: Serban Iorga <serban@parity.io>
Co-authored-by: Chevdor <chevdor@users.noreply.github.com>
Co-authored-by: Egor_P <egor@parity.io>
Co-authored-by: Bastian Köcher <git@kchr.de>

## 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>