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>

polkadot/README.md

@@ -212,7 +212,7 @@ that we currently maintain.
 
 ### Using Docker
 
-[Using Docker](../docs/docker.md)
+[Using Docker](../docs/contributor/docker.md)
 
 ### Shell Completion
 
@@ -222,11 +222,11 @@ that we currently maintain.
 
 ### Contributing Guidelines
 
-[Contribution Guidelines](https://github.com/paritytech/polkadot-sdk/blob/master/docs/CONTRIBUTING.md)
+[Contribution Guidelines](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md)
 
 ### Contributor Code of Conduct
 
-[Code of Conduct](https://github.com/paritytech/polkadot-sdk/blob/master/docs/CODE_OF_CONDUCT.md)
+[Code of Conduct](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CODE_OF_CONDUCT.md)
 
 ## License