Ref doc list

[juangirini]

Document the following items, listed here

### Subtasks
- [x] `trait_based_programming`
- [x] `blockchain_state_machines`
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/40
- [x] `wasm_meta_protocol`
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/41
- [x] `extrinsic_encoding`
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/42
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/43
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/44
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/45
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/46
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/47
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/48
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/49
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/50
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/51
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/52
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/53
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/54
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/55
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/74
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/73
- [ ] https://github.com/paritytech/polkadot-sdk-docs/issues/71

[nuke-web3]

For the (at this time) unassigned mods, there are PBA materials that could act as the basis for crafting each doc item/mod, perhaps cross referencing the PBA book so there is less duplication of information:

• trait_based_programming - There is a private "assignment 0" that covers more than https://github.com/Polkadot-Blockchain-Academy/pba-qualifier-exam that is a good reference for non-trivial rust traits. @kianenigma may be able to open that up for this content and the next one, as it covers an impl of a basic cryptocurrency in the style of Substrate. See also tips and tricks for key topics to consider in this doc item.
• blockchain_state_machines - The PBA entrance exam has a minimal impl as reference to pull from perhaps, and the content from the BA cohort might serve as a good basis to reference, and the most recent book entry should be used as the most trusted up-to-date on that content.
• wasm_meta_protocol The slides and associated crate for WASM here can be used as a solid basis.
• extrinsic_encoding - SCALE from PBA is a good conceptual start. Unclear if this doc item should cover more than just SCALE...?

Edit: I see these do now have a start in most cases here: https://github.com/paritytech/polkadot-sdk/tree/master/docs/sdk/src/reference_docs (vs the OP linked commit to a now too old version 😅 )