docs: autogenerate compiler flag stubs based on -Zhelp

[rmehri01]

Adds autogenerated compiler flag stubs to the unstable book by building rustc, passing it's path to unstable-book-gen, and using that to call the compiler with -Zhelp and create a similar Features that is used for library and lang stubs.

Example:

# `combine_cgu`

combine CGUs into a single one

This feature has no tracking issue, and is therefore likely internal to the compiler, not being intended for general use.

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Closes rust-lang/rust#141525