An attempt to improve [the
docs](https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/index.html)
by applying various corrections:

- grammar/stylistics,
- formatting,
- broken links,
- broken markdown table,
- outdated vscode setting name,
- typos,
- consistency,
- etc.

Part of https://github.com/paritytech/eng-automation/issues/10

Co-authored-by: Bastian Köcher <git@kchr.de>

cc https://github.com/paritytech/ci_cd/issues/974

---------

Co-authored-by: command-bot <>
Co-authored-by: Bastian Köcher <info@kchr.de>

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>

cc https://github.com/paritytech/ci_cd/issues/943

A brief explanation of the Deprecation Checklist is added to the
Contributing notes with a link to it

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>