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/frame_benchmarking_weight.rs

@@ -0,0 +1,23 @@
+//! # FRAME Benchmarking and Weights.
+//!
+//! Notes:
+//!
+//! On Weight as a concept.
+//!
+//! - Why we need it. Super important. People hate this. We need to argue why it is worth it.
+//! - Axis of weight: PoV + Time.
+//! - pre dispatch weight vs. metering and post dispatch correction.
+//! 	- mention that we will do this for PoV
+//! 	- you can manually refund using `DispatchResultWithPostInfo`.
+//! - Technically you can have weights with any benchmarking framework. You just need one number to
+//!   be computed pre-dispatch. But FRAME gives you a framework for this.
+//! - improve documentation of `#[weight = ..]` and `#[pallet::weight(..)]`. All syntax variation
+//!   should be covered.
+//!
+//! on FRAME benchmarking machinery:
+//!
+//! - component analysis, why everything must be linear.
+//! - how to write benchmarks, how you must think of worst case.
+//! - how to run benchmarks.
+//!
+//! - https://www.shawntabrizi.com/substrate/substrate-storage-deep-dive/