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

This PR is a continuation of
paritytech#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 paritytech#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>

substrate/README.md

@@ -3,7 +3,7 @@
 [![GitHub license](https://img.shields.io/badge/license-GPL3%2FApache2-blue)](#LICENSE)
 [![GitLab
 Status](https://gitlab.parity.io/parity/mirrors/polkadot-sdk/badges/master/pipeline.svg)](https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/pipelines)
-[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/CONTRIBUTING.md)
+[![PRs Welcome](https://img.shields.io/badge/PRs-welcome-brightgreen.svg)](docs/contributor/CONTRIBUTING.md)
 [![Stack
 Exchange](https://img.shields.io/badge/Substrate-Community%20&%20Support-24CC85?logo=stackexchange)](https://substrate.stackexchange.com/)
 <p align="center">
@@ -26,15 +26,13 @@ here](https://github.com/paritytech/polkadot-sdk/issues) for anything you suspec
 
 ## Contributions & Code of Conduct
 
-Please follow the contributions guidelines as outlined in
-[`docs/CONTRIBUTING.md`](https://github.com/paritytech/polkadot-sdk/blob/master/docs/CONTRIBUTING.md). In all
-communications and contributions, this project follows the [Contributor Covenant Code of
-Conduct](https://github.com/paritytech/polkadot-sdk/blob/master/docs/CODE_OF_CONDUCT.md).
+Please follow the contributions guidelines as outlined in [`docs/contributor/CONTRIBUTING.md`](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md).
+In all communications and contributions, this project follows the [Contributor Covenant Code of Conduct](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CODE_OF_CONDUCT.md).
 
 ## Security
 
 The security policy and procedures can be found in
-[`docs/SECURITY.md`](https://github.com/paritytech/polkadot-sdk/blob/master/docs/SECURITY.md).
+[`docs/contributor/SECURITY.md`](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/SECURITY.md).
 
 ## License
 

substrate/frame/src/lib.rs

@@ -32,7 +32,7 @@
 //!
 //! ## Documentation
 //!
-//! See [`polkadot_sdk::frame`](../developer_hub/polkadot_sdk/frame_runtime/index.html).
+//! See [`polkadot_sdk::frame`](../polkadot_sdk_docs/polkadot_sdk/frame_runtime/index.html).
 //!
 //! ## Warning: Experimental
 //!