Adjust header in refine/

New header links: "Understand", "Use", "Contribute", "Community"

These are more succinct than the links proposed in #307 while (arguably)
retaining clarity and cohesion.  If this change is accepted, we ought to
update links containing "support" to "contribute" to maintain
consistency.
Merge pull request #311 from confit/master

Adjust header in refine/