libstdc++: Update docs on build process and generated files

There are several more sub-directories below 'src' now, with lots more
conveience libraries. Document them all as of GCC 14.

Also document how to regenerate the generated headers under include/bits
and how to update the tzdata.zi file.

libstdc++-v3/ChangeLog:

	* doc/xml/manual/build_hacking.xml: Document generated files.
	Update list of convenience libraries and sub-directories under
	the src directory.
	* doc/html/*: Regenerate.