Create sections inside the unstable book

[est31]

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.

[frewsxcv]

Folding this into #41142

[frewsxcv]

Or if you feel that issue is different than what you're proposing, feel free to reopen.

[frewsxcv]

I guess it's sufficiently different.

[est31]

Oh so you made PR #41295 ... only discovered it now. It has rougher categories than what was suggested by me, but I think its a good compromise. Thanks! Closing.