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/markdown_linting.md

@@ -0,0 +1,20 @@
+# Markdown linting
+
+Since the introduction of [PR #1309](https://github.com/paritytech/polkadot-sdk/pull/1309), the markdown
+files in this repository are checked by a linter for formatting and consistency.
+
+The linter used is [`markdownlint`](https://github.com/DavidAnson/markdownlint) and can be installed locally on your
+machine. It can also be setup as [pre-commit hook](https://github.com/igorshubovych/markdownlint-cli#use-with-pre-commit)
+to ensure that your markdown is passing all the tests.
+
+The rules in place are defined
+[here](https://github.com/paritytech/polkadot-sdk/blob/master/.github/.markdownlint.yaml).
+
+You may run `markdownlint` locally using:
+```
+markdownlint --config .github/.markdownlint.yaml --ignore target .
+```
+
+There are also plugins for your favorite editor, that can ensure that most
+of the rules will pass and fix typical issues (such as trailing spaces,
+missing eof new line, long lines, etc...)