Stabilize rustdoc `doc_cfg` feature

[nikomatsakis]

Metadata
Point of contact	@GuillaumeGomez
Team(s)	rustdoc
Goal document	2025h2/rustdoc-doc-cfg

Summary

Over the next six months, we will do the following:

• Finish the review and implementation of the feature.
• Get users feedback.
• Stabilize the feature.

Tasks and status

• Standard reviews (rustdoc )
• Communicate about the feature (@GuillaumeGomez)
• Enable it by default on docs.rs (@GuillaumeGomez)
• Send pull request to stabilize the feature (@GuillaumeGomez)

[nikomatsakis]

This issue is intended for status updates only.

For general questions or comments, please contact the owner(s) directly.

[GuillaumeGomez]

Opened stabilization PR but we have blockers I didn't hear of, so stabilization will be postponed until then.

[nxsaken]

The 2025h2 goal period is now over, so I'm closing this issue. Thanks to everyone who contributed! Authors are welcome to leave a final comment about the goal's outcome.