Polkadot OmniNode Docs (#6094)

provides low-level documentation on how the omni-node is meant to work.
This is meant to act as reusable material for other teams (e.g.
Papermoon and W3F) to use and integrate into the high level Polkadot
documentation.

Broadly speaking, for omni-node to have great rust-docs, we need to
focus on the following crates, all of which got a bit of love in this
PR:

1. `sp-genesis-builder`
2. `polkadot-omni-node`
3. `polkadot-omni-node-lib`
4. `frame-omni-bencher`

On top of this, we have now: 

* `polkadot_sdk_docs::guides` contains two new steps demonstrating the
most basic version of composing your pallet, putting it into a runtime,
and putting that runtime into omni-node
* `polkadot_sdk_docs::reference_docs::omni_node` to explain in more
detail how omni-node differs from the old-school node.
* `polkadot_sdk_docs::reference_docs::frame_weight_benchmarking` to
finally have a minimal reference about weights and benchmarking.
* It provides tests for some of the steps in
https://github.com/paritytech/polkadot-sdk/issues/5568


closes https://github.com/paritytech/polkadot-sdk/issues/5568
closes https://github.com/paritytech/polkadot-sdk/issues/4781

Next steps

- [x] Ensure the README of the parachain template is up-to-date.
@iulianbarbu


- [ ] Readme for `polkadot-omni-node` and similar is updated. For now,
use `cargo-readme` and copy over the rust-docs.

To build the branch locally and run this:
https://paritytech.github.io/polkadot-sdk/master/polkadot_sdk_docs/meta_contributing/index.html#how-to-develop-locally

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Co-authored-by: Iulian Barbu <[email protected]>
Co-authored-by: Sebastian Kunert <[email protected]>
Co-authored-by: Michal Kucharczyk <[email protected]>