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/prdoc.md

@@ -0,0 +1,71 @@
+# PRDoc
+
+## Intro
+
+With the merge of [PR #1946](https://github.com/paritytech/polkadot-sdk/pull/1946), a new method for
+documenting changes has been introduced: `prdoc`. The [prdoc repository](https://github.com/paritytech/prdoc)
+contains more documentation and tooling.
+
+The current document describes how to quickly get started authoring `PRDoc` files.
+
+## Requirements
+
+When creating a PR, the author needs to decides with the `R0` label whether the change (PR) should
+appear in the release notes or not.
+
+Labelling a PR with `R0` means that no `PRDoc` is required.
+
+A PR without the `R0` label **does** require a valid `PRDoc` file to be introduced in the PR.
+
+## PRDoc how-to
+
+A `.prdoc` file is a YAML file with a defined structure (ie JSON Schema).
+
+For significant changes, a `.prdoc` file is mandatory and the file must meet the following
+requirements:
+- file named `pr_NNNN.prdoc` where `NNNN` is the PR number.
+  For convenience, those file can also contain a short description: `pr_NNNN_foobar.prdoc`.
+- located under the [`prdoc` folder](https://github.com/paritytech/polkadot-sdk/tree/master/prdoc) of the repository
+- compliant with the [JSON schema](https://json-schema.org/) defined in `prdoc/schema_user.json`
+
+Those requirements can be fulfilled manually without any tooling but a text editor.
+
+## Tooling
+
+Users might find the following helpers convenient:
+- Setup VSCode to be aware of the prdoc schema: see [using VSCode](https://github.com/paritytech/prdoc#using-vscode)
+- Using the `prdoc` cli to:
+  - generate a `PRDoc` file from a [template defined in the Polkadot SDK
+    repo](https://github.com/paritytech/polkadot-sdk/blob/master/prdoc/.template.prdoc) simply providing a PR number
+  - check the validity of one or more `PRDoc` files
+
+## `prdoc` cli usage
+
+The `prdoc` cli documentation can be found at https://github.com/paritytech/prdoc#prdoc
+
+tldr:
+- `prdoc generate <NNNN>`
+- `prdoc check -n <NNNN>`
+
+where <NNNN> is the PR number.
+
+## Pick an audience
+
+While describing a PR, the author needs to consider which audience(s) need to be addressed.
+The list of valid audiences is described and documented in the JSON schema as follow:
+
+- `Node Dev`: Those who build around the client side code. Alternative client builders, SMOLDOT, those who consume RPCs.
+   These are people who are oblivious to the runtime changes. They only care about the meta-protocol, not the protocol
+   itself.
+
+- `Runtime Dev`: All of those who rely on the runtime. A parachain team that is using a pallet. A DApp that is using a
+   pallet. These are people who care about the protocol (WASM), not the meta-protocol (client).
+
+- `Node Operator`: Those who don't write any code and only run code.
+
+- `Runtime User`: Anyone using the runtime. This can be a token holder or a dev writing a front end for a chain.
+
+## Tips
+
+The PRDoc schema is defined in each repo and usually is quite restrictive.
+You cannot simply add a new property to a `PRDoc` file unless the Schema allows it.