Gut install.html

[Benjamin Kosnik]

>This rips out all the instructions for how to set up separate source and
>build trees, the commands to issue, and so forth.  Reactions?

Looks great. Thanks for taking care of this!

The structuring of the libstdc++ docs is a longer-term issue and concern
of mine. At some point, I'm hoping some excellent solution will just
become blindingly obvious. Till then I guess it's just writing html and
hand editing the various links so that it hangs together.

-benjamin