Changes:
- Make R0-silent not run the semver check again. Originally I thought
this would be good to have a bullet-proof check, but it now often
triggers when CI or unrelated files are changed. In the end, the
developer has to make the right choice here - and always will need to.
So bringing back the R0 label gives more power to the devs and should
increase dev velocity. We still need to ensure that every use of this
label is well understood, and not just used out of lazyness.
- Fix `/cmd prdoc` bump levels
- Update docs

---------

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

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>