Fix Mermaid diagram rendering (#3875)

Closes https://github.com/paritytech/polkadot-sdk/issues/2977

The issue appears to stem from the `aquamarine` crate failing to render
diagrams in re-exported crates.

e.g. as raised
[here](https://github.com/paritytech/polkadot-sdk/issues/2977), diagrams
would render at `frame_support::traits::Hooks` but not the re-exported
doc `frame::traits::Hooks`, even if I added `aquamarine` as a `frame`
crate dependency.

To resolve this, I followed advice in
https://github.com/mersinvald/aquamarine/issues/20 to instead render
mermaid diagrams directly using JS by adding an `after-content.js`.

---

Also fixes compile warnings, enables `--all-features` and disallows
future warnings in CI.

---------

Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Bastian Köcher <git@kchr.de>

cumulus/test/service/src/chain_spec.rs

@@ -34,7 +34,7 @@ pub fn get_from_seed<TPublic: Public>(seed: &str) -> <TPublic::Pair as Pair>::Pu
 		.public()
 }
 
-/// The extensions for the [`ChainSpec`](crate::ChainSpec).
+/// The extensions for the [`ChainSpec`].
 #[derive(Debug, Clone, PartialEq, Serialize, Deserialize, ChainSpecGroup, ChainSpecExtension)]
 #[serde(deny_unknown_fields)]
 pub struct Extensions {