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/reference_docs/blockchain_state_machines.rs

@@ -0,0 +1,29 @@
+//! # State Transition Function
+//!
+//! This document briefly explains how in the context of Substrate-based blockchains, we view the
+//! blockchain as a **decentralized state transition function**.
+//!
+//! Recall that a blockchain's main purpose is to help a permissionless set of entities to agree on
+//! a shared data-set, and how it evolves. This is called the **State**, also referred to as
+//! "onchain" data, or *Storage* in the context of FRAME. The state is where the account balance of
+//! each user is, for example, stored, and there is a canonical version of it that everyone agrees
+//! upon.
+//!
+//! Then, recall that a typical blockchain system will alter its state through execution of blocks.
+//! *The component that dictates how this state alteration can happen is called the state transition
+//! function*.
+#![doc = simple_mermaid::mermaid!("../../../mermaid/stf_simple.mmd")]
+//!
+//! In Substrate-based blockchains, the state transition function is called the *Runtime*. This is
+//! explained further in [`crate::reference_docs::wasm_meta_protocol`].
+//!
+//! With this in mind, we can paint a complete picture of a blockchain as a state machine:
+#![doc = simple_mermaid::mermaid!("../../../mermaid/stf.mmd")]
+//!
+//! In essence, the state of the blockchain at block N is the outcome of applying the state
+//! transition function to the the previous state, and the current block as input. This can be
+//! mathematically represented as:
+//!
+//! ```math
+//! STF = F(State_N, Block_N) -> State_{N+1}
+//! ```