Tracking Issue for user-provided classnames in fenced code blocks

[poliorcetics]

The feature gate for the issue is #![feature(custom_code_classes_in_docs)].

About tracking issues

Tracking issues are used to record the overall progress of implementation.
They are also used as hubs connecting to other relevant issues, e.g., bugs or open design questions.
A tracking issue is however not meant for large scale discussion, questions, or bug reports about a feature.
Instead, open a dedicated issue for the specific matter and add the relevant feature gate label.

Steps

• Implement the feature (Accept additional user-defined syntax classes in fenced code blocks #110800)
• Adjust documentation (Also done in Accept additional user-defined syntax classes in fenced code blocks #110800)
• Stabilization PR (Stabilize custom_code_classes_in_docs feature #124577) (see instructions on rustc-dev-guide)

Unresolved Questions

XXX --- list all the "unresolved questions" found in the RFC to ensure they are
not forgotten

Implementation history

• Feature implementation, WIP: Accept additional user-defined classes in fenced code blocks #79454

@rustbot label T-rustdoc