Add documentation around FRAME Origin (#3362)

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 <86818441+DrW3RK@users.noreply.github.com>
Co-authored-by: Sebastian Kunert <skunert49@gmail.com>
Co-authored-by: Gonçalo Pestana <g6pestana@gmail.com>
Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>

.gitlab/pipeline/build.yml

@@ -91,7 +91,7 @@ build-rustdoc:
     - .run-immediately
   variables:
     SKIP_WASM_BUILD: 1
-    RUSTDOCFLAGS: ""
+    RUSTDOCFLAGS: "--default-theme=ayu --html-in-header ./docs/sdk/headers/header.html --extend-css ./docs/sdk/headers/theme.css"
   artifacts:
     name: "${CI_JOB_NAME}_${CI_COMMIT_REF_NAME}-doc"
     when: on_success