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>

docs/sdk/src/lib.rs

@@ -15,7 +15,7 @@
 //! - Start by learning about the the [`polkadot_sdk`], its structure and context.
 //! - Then, head over the [`guides`]. This modules contains in-depth guides about the most important
 //!   user-journeys of the Polkadot SDK.
-//! - Whilst reading the guides, you might find back-links to [`crate::reference_docs`].
+//! - Whilst reading the guides, you might find back-links to [`reference_docs`].
 //! - Finally, <https://paritytech.github.io> is the parent website of this crate that contains the
 //!   list of further tools related to the Polkadot SDK.
 //!
@@ -25,6 +25,11 @@
 #![doc = simple_mermaid::mermaid!("../../mermaid/IA.mmd")]
 #![warn(rustdoc::broken_intra_doc_links)]
 #![warn(rustdoc::private_intra_doc_links)]
+#![doc(html_favicon_url = "https://polkadot.network/favicon-32x32.png")]
+#![doc(
+	html_logo_url = "https://europe1.discourse-cdn.com/standard21/uploads/polkadot2/original/1X/eb57081e2bb7c39e5fcb1a98b443e423fa4448ae.svg"
+)]
+#![doc(issue_tracker_base_url = "https://github.com/paritytech/polkadot-sdk/issues")]
 
 /// Meta information about this crate, how it is built, what principles dictates its evolution and
 /// how one can contribute to it.