Sites docs

This is the first time I have used this functionality, so several caveats:

* The last section, about SSL, has an unresolved issue about SSL_FORCE_HOST, so is marked "PERHAPS OMIT".

* I didn't actually test with the subdomains example.

Also, I followed the theming tutorials by Josh Cartmell, specifically the part about building the HomePage content type: http://bitofpixels.com/blog/mezzatheming-part-2-the-homepage/, and read mailing list discussions about the sharing of base.html, compared to a potentially better solution (would require enhancing / replacing the HOST_THEMES approach).