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>

.gitlab/pipeline/build.yml

@@ -91,7 +91,7 @@ build-rustdoc:
     - .run-immediately
   variables:
     SKIP_WASM_BUILD: 1
-    RUSTDOCFLAGS: "--default-theme=ayu --html-in-header ./docs/sdk/headers/header.html --extend-css ./docs/sdk/headers/theme.css"
+    RUSTDOCFLAGS: "-Dwarnings --default-theme=ayu --html-in-header ./docs/sdk/assets/header.html --extend-css ./docs/sdk/assets/theme.css --html-after-content ./docs/sdk/assets/after-content.html"
   artifacts:
     name: "${CI_JOB_NAME}_${CI_COMMIT_REF_NAME}-doc"
     when: on_success
@@ -99,8 +99,7 @@ build-rustdoc:
     paths:
       - ./crate-docs/
   script:
-    # FIXME: it fails with `RUSTDOCFLAGS="-Dwarnings"` and `--all-features`
-    - time cargo doc --features try-runtime,experimental --workspace --no-deps
+    - time cargo doc --all-features --workspace --no-deps
     - rm -f ./target/doc/.lock
     - mv ./target/doc ./crate-docs
     # Inject Simple Analytics (https://www.simpleanalytics.com/) privacy preserving tracker into