documentation and new --enable flags

[Benjamin Kosnik]

>    - names of, and links to, other documentation files, including the
>      RELEASE-NOTES and the FAQ, and stuff under docs/17_intro/
>    - install
>    - usage
>    - explanation of the funkier 'configure' options
> 
> The --enable-libgcc-rebuild is hairy enough that it got its own little
> page, pointed to from the explanation list above.

sounds good

> This can replace the README.html file directly (my vote), or we can name
> this one documentation.html and remove README.html.  It's essentially the
> same thing, with a view to the larger organization.  The "sidebar" on the
> main page can point to this as something more official than the current
> "How to install the library," and the other pages can take that into account.

sure. I'm not going to get picky about file naming right now . . .

> I'd like a way to feed other -f[unky] options in as well.  Right now the
> only way is to redefine CXXFLAGS, but then we lose when rebuilding libgcc.a.
> I suggest

can the rebuild of libgcc.a take into account CXXFLAGS?

If you can, please check in your work for the rebuild before starting on
this. . . I'm eager to see how it works.

-benjamin