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`.

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

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Bastian Köcher <[email protected]>