[code review] open source docs

[Jeff Kaufman]

Josh,

Could you review this change:
https://github.com/pagespeed/mod_pagespeed/pull/1459

This is an open source version of the user-facing docs on
developers.google.com/speed/pagespeed/module/. I currently have it
staged at modpagespeed.com/doc, which is probably where this should go
when live.

It uses server-side includes for headers and footers, which isn't as
nice as being fully static, but should be more maintainable.

This change doesn't include a landing page for mod_pagespeed. I think
that should probably go on modpagespeed.com / ngxpagespeed.com and not
in the docs directly. Having it in the docs is a weird artifact of how
we were hosted on developers.google.com. But this is not in this CL.

Jeff