rustdoc: Tracking issue for `#[doc(cfg)]` and automatically documented `#[cfg]`s

[kennytm]

This is a tracking issue for RFC 3631.
The feature gate for this doc_cfg (doc_cfg_hide and doc_auto_cfg were removed in favor this overarching one).

Part of #1998.

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

• Experimental implementation
• Adjust documentation
• Write an RFC
• Update implementation to match RFC
• Check performance impact on big crates
• Stabilization PR (see instructions on forge):

Implementation history

• Expose all OS-specific modules in libstd doc. #43348
• Unify rustc and rustdoc parsing of cfg() #84442
• Make cfg imply doc(cfg) #89596
• Split doc_cfg and doc_auto_cfg features #90502
• Update documentation for doc_cfg feature #92818
• Implement RFC 3631: add rustdoc doc_cfg features #138907