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>

docs/sdk/assets/after-content.html

@@ -0,0 +1,2 @@
+<script> mermaid.init({ startOnLoad: true, theme: "dark" }, "pre.language-mermaid > code");</script>
+

docs/sdk/assets/header.html

@@ -84,6 +84,8 @@
 	});
 </script>
 
+<script src="https://cdn.jsdelivr.net/npm/mermaid/dist/mermaid.min.js"></script>
+
 <style>
 	body.sdk-docs {
 		nav.side-bar {