Does the following: 

- Add a reference doc page named `frame_runtime_types`, which explains
what types like `RuntimeOrigin`, `RuntimeCall` etc are.
- On top of it, it adds a reference doc page called `frame_origin` which
explains a few important patterns that we use around origins
- And finally brushes up `#[frame::origin]` docs. 
- Updates the theme, sidebar and favicon to look like: 

<img width="1728" alt="Screenshot 2024-02-20 at 12 16 00"
src="https://github.com/paritytech/polkadot-sdk/assets/5588131/6d60a16b-2081-411b-8869-43b91920cca9">


All of this was inspired by
https://substrate.stackexchange.com/questions/10992/how-do-you-find-the-public-key-for-the-medium-spender-track-origin/10993

closes https://github.com/paritytech/polkadot-sdk-docs/issues/45
closes https://github.com/paritytech/polkadot-sdk-docs/issues/43
contributes / overlaps with
https://github.com/paritytech/polkadot-sdk/pull/2638 cc @liamaharon


deprecation companion:
https://github.com/substrate-developer-hub/substrate-docs/pull/2131
pba-content companion:
https://github.com/Polkadot-Blockchain-Academy/pba-content/pull/977

---------

Co-authored-by: Radha <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: Liam Aharon <[email protected]>