add CF FAQ to CF Conventions web site

This pull request encompasses the changes needed to add a FAQ to the CF Conventions web site:

1) Add the faq.md itself
2) Change the home page to point to the FAQ
3) Change the default.html in the _layouts template so that jumps to anchors end up in the right place

There are a few answers that could use some refinement, but this is noted in context and overall it is ready to go-go-go. (Note Jonathan reviewed a slightly older version of the FAQ. I've addressed his comments in this version.)