[libstdc++] automate documentation.html changes, basic_string doxygen hooks

[Benjamin Kosnik]

> Benjmain recently copied out all of the "Contents" blocks from our
> various HOWTOs into an expanded form in the parent documentation.html.
> To avoid repetitious editing, this makes documentation.html a target in
> docs/html/Makefile.  So, for example, after editing 21_strings/howto.html
> to add a new entry, it automatically showed up in the expanded list.

Yes! Thanks. I could not figure out how to do this. I'm glad you did.
Having to regenerate all that stuff by hand was going to be a pain in
the neck.

-benjamin