Add list of current unstable APIs to docs

[epompeii]

The motivation behind making this change is to increase the discoverability of unstable APIs. The current list has been populated using the contents of wasm-bindgen/crates/web-sys/webidls/unstable/. Going forward, MRs that add or remove unstable APIs will need to update this list accordingly.

[alexcrichton]

Is there a way this list could be auto-generated? I otherwise will surely forget to ever remind folks to update this when WebIDL changes.

[epompeii]

Yeah, that would be best! One option is to create an mdbook preprocessor that auto-generates the markdown list based on a given file path. Or I could add a check in the doc_book CI step that validates the list against the contents of wasm-bindgen/crates/web-sys/webidls/unstable/. At the very least this would catch a missed update and prompt the appropriate change.

[alexcrichton]

Either way is fine by me, either generating this during the doc build or validating on CI the list is correct. Whichever is easiest to compute seems fine by me!

[daxpedda]

Automating this should be easily implementable in the Webidl generator.