doc: Document <stdbit.h> function-like macros.
* doc/posix-headers/stdbit.texi: Reference ISO C 23 as the
documentation. Update platforms list.
* doc/posix-functions/stdc_leading_zeros.texi: New file.
* doc/posix-functions/stdc_leading_ones.texi: New file.
* doc/posix-functions/stdc_trailing_zeros.texi: New file.
* doc/posix-functions/stdc_trailing_ones.texi: New file.
* doc/posix-functions/stdc_first_leading_zero.texi: New file.
* doc/posix-functions/stdc_first_leading_one.texi: New file.
* doc/posix-functions/stdc_first_trailing_zero.texi: New file.
* doc/posix-functions/stdc_first_trailing_one.texi: New file.
* doc/posix-functions/stdc_count_zeros.texi: New file.
* doc/posix-functions/stdc_count_ones.texi: New file.
* doc/posix-functions/stdc_has_single_bit.texi: New file.
* doc/posix-functions/stdc_bit_width.texi: New file.
* doc/posix-functions/stdc_bit_floor.texi: New file.
* doc/posix-functions/stdc_bit_ceil.texi: New file.
* doc/gnulib.texi (Function Substitutes): Include them all.