add trademark policy at /trademark

This policy was written by Software Freedom Conservancy's
legal counsel on behalf of the Git Project. It needs to be
posted publicly; this patch starts by making it available,
and we'll link to it in future commits.
about: link to trademark page from "free and open source"

This seems like the most sensible place to link to it.
layouts: only use "site" layout for site#index

The /sfc page is rendered with the masthead and no navbar,
just like the main page. This makes it hard to read, and
hard to navigate away from. This is likely an unintended
consequence of c575cbd.

Let's only use that layout for the index page. All of the
other pages handled by the site controller (/sfc,
/courses/svn, and the search results page) probably want to
be treated more like "regular" pages.

Fixes #388.
Merge pull request #400 from git/peff/fix-sfc-layout

layouts: only use "site" layout for site#index
Merge pull request #401 from git/peff/trademark-policy

add trademark policy