rustdoc: Tracking issue for command-line flag `--output-format=doctest`

This option is needed by the Rust-for-Linux project. The goal is to allow them to extract doctests so they can modify them and run them however they want to simplify their testing pipeline.

[!NOTE]

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.
Instead, open a dedicated issue for the specific matter or post in the #t-rustdoc Zulip channel.

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Steps

• Implement the feature
• Stabilization PR (see instructions on rustc-dev-guide)

Unresolved Questions

• Should switch from --output-format doctest to --emit doctest? See also https://github.com/rust-lang/rust/issues/83784#issuecomment-3572830043

Implementation history

• https://github.com/rust-lang/rust/pull/134531
• https://github.com/rust-lang/rust/pull/141399

Start with the tracking checklist and the stabilization guide, then read implementation PRs #134531 and #141399. Review the unresolved question about --output-format doctest versus --emit doctest, including issue #83784. Done means the stabilization work is completed and the flag naming question is resolved.