Autogenerate unstable compiler flag stubs for unstable-book

[kohsine]

Closes #141525.

Based on old PR #142135.

Parses rustc -Zhelp to generate doc stubs for unstable compiler flags if any are missing.

[rustbot]

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[clubby789]

For context, a previous attempt at this parsed the options macro: #154070 This was reverted in #154488. Would you be interested in investigating an approach that addresses the concerns raised?

If not, please add a Co-authored-by: credit with the original author of this code to your commit.