Document other kinds of unstability in The Unstable Book

[frewsxcv]

Right now, The Unstable Book is just a list of unstable lang/lib features. There are other things like compiler flags which are also considered unstable that should be documented in The Unstable Book. This requires adjusting the tidy lint a bit to accommodate other sections. Right now I'm thinking that all sections (features, flags, etc) will live in their own subdirectory.

[frewsxcv]

Relevant thread: #40018 (comment)

[frewsxcv]

Reminder for future self: remove this hack e280515

[brson]

This may not be the issue, but there are a number of stable features that are actually just "stable", like target specs and llvm plugins, where we don't even try to maintain stability. It would be nice to have a discussion of those somewhere.

[frewsxcv]

I'm waiting on #40018 to merge before I work on this.

[frewsxcv]

Copying @est31's text from #41289:

I think for greater categorisation, the unstable book should be divided into sections.

A proposal:

• Compiler internal lang features ("not intended for general use")
• Permanently unstable features that are not compiler internal (like the llvm intrinsics)
• Novel lang features (with intent to stabilize them some day)
• Library features
• ??

Some sections should have a higher standard of documentation, like the novel lang features, others like the library features already have documentation in rustdoc (see #41288), so their documentation should be limited to a link to the tracking issue (and maybe their rustdoc), and the "not intended for general use" features can get a lower priority to be documented as well.