[docs] Rename to permit wider variety of  content.

I don't want to rule out the possibility of linking to e.g. interesting
blog posts about uses of Clang, so avoid restricting the content to
"projects".

This breaks URL compatibility, but this document was committed less than
an hour ago so hopefully nobody has linked to it yet.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@175535 91177308-0d34-0410-b5e6-96231b3b80d8