Tracking issue for `#[doc(cfg(…))]`, `#[doc(cfg_hide(…))]` and `doc_auto_cfg`

[kennytm]

This is a tracking issue for the #[doc(cfg(…))] attribute (feature: doc_cfg) introduced in #43348 and #[doc(cfg_hide(…))] (feature: doc_cfg_hide) attribute introduced in #89596, along with the doc_auto_cfg feature introduced in #90502.

Steps:

• Implement: Expose all OS-specific modules in libstd doc. #43348
• Fix syntax mismatches: Rustdoc and rustc accept different cfg syntax #84437 (fixed in Unify rustc and rustdoc parsing of cfg() #84442)
• Adjust documentation (see instructions on forge) (Update documentation for doc_cfg feature #92818)
• RFC (RFC for doc_cfg, doc_cfg_auto, doc_cfg_hide and doc_cfg_show features rfcs#3631)
• Update implementation to match RFC (Implement RFC 3631: add rustdoc doc_cfg features #138907)
• Check performance impact on big crates
• Update documentation to match RFC
• Stabilization PR (see instructions on forge):

(cc #1998)