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/sdk/src/guides/mod.rs

@@ -0,0 +1,25 @@
+//! # Polkadot SDK Docs Guides
+//!
+//! This crate contains a collection of guides that are foundational to the developers of
+//! Polkadot SDK. They common user-journeys that are traversed in the Polkadot ecosystem.
+
+/// Write your first simple pallet, learning the most most basic features of FRAME along the way.
+pub mod your_first_pallet;
+
+/// Writing your first real [runtime](`crate::reference_docs::wasm_meta_protocol`), and successfully
+/// compiling it to [WASM](crate::polkadot_sdk::substrate#wasm-build).
+pub mod your_first_runtime;
+
+/// Running the given runtime with a node. No specific consensus mechanism is used at this stage.
+pub mod your_first_node;
+
+/// How to change the consensus engine of both the node and the runtime.
+pub mod changing_consensus;
+
+/// How to enhance a given runtime and node to be cumulus-enabled, run it as a parachain and connect
+/// it to a relay-chain.
+pub mod cumulus_enabled_parachain;
+
+/// How to make a given runtime XCM-enabled, capable of sending messages (`Transact`) between itself
+/// and the relay chain to which it is connected.
+pub mod xcm_enabled_parachain;