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>

docs/contributor/PULL_REQUEST_TEMPLATE.md

@@ -0,0 +1,37 @@
+
+
+✄ -----------------------------------------------------------------------------
+
+Thank you for your Pull Request! 🙏 Please make sure it follows the contribution guidelines outlined in
+[this document](https://github.com/paritytech/polkadot-sdk/blob/master/docs/contributor/CONTRIBUTING.md) and fill
+out the sections below. Once you're ready to submit your PR for review, please
+delete this section and leave only the text under the "Description" heading.
+
+# Description
+
+*Please include a summary of the changes and the related issue. Please also include relevant motivation and context,
+including:*
+
+- What does this PR do?
+- Why are these changes needed?
+- How were these changes implemented and what do they affect?
+
+*Use [Github semantic
+linking](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword)
+to address any open issues this PR relates to or closes.*
+
+Fixes # (issue number, *if applicable*)
+
+Closes # (issue number, *if applicable*)
+
+# Checklist
+
+- [ ] My PR includes a detailed description as outlined in the "Description" section above
+- [ ] My PR follows the [labeling requirements](CONTRIBUTING.md#Process) of this project (at minimum one label for `T`
+  required)
+- [ ] I have made corresponding changes to the documentation (if applicable)
+- [ ] I have added tests that prove my fix is effective or that my feature works (if applicable)
+
+You can remove the "Checklist" section once all have been checked. Thank you for your contribution!
+
+✄ -----------------------------------------------------------------------------