Move developer-hub to polkadot-sdk-docs (#2598)

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>

.github/workflows/check-prdoc.yml

@@ -12,7 +12,7 @@ env:
   GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}
   GITHUB_PR: ${{ github.event.pull_request.number }}
   ENGINE: docker
-  PRDOC_DOC: https://github.com/paritytech/polkadot-sdk/blob/master/docs/prdoc.md
+  PRDOC_DOC: https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/prdoc.md
 
 jobs:
   check-prdoc: