[libc][docs] use docgen to generate stdbit rst docs (#88540)

Closes #88066.

Compared to before, the function names in the stdbit table are sorted by
function name, not order-of-appearance in the standard. Since macros
aren't printed by docgen.py and are still a TODO in the code, they are
also not printed in the new stdbit.h docs.

Adds some checks to docgen.py for conditions that tripped me up.

Add code to docgen.py to add the include of the `|check|` rewriter,
since all other generated files need it.