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 <[email protected]>
Co-authored-by: Chevdor <[email protected]>
Co-authored-by: Egor_P <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>