Update PoC wording and docs links on install page
Update PoC wording and docs links on install page

Now that the new documentation structure has gone live, update the
"Proof of Concept" language on the install page to instead say
"Evaluation" and adjust the documentation links so they reflect
the new structure, and aren't pointing at URLs which are now
being redirected.

dcos/dcos-website,dcos/dcos-website,dcos/dcos-website,dcos/dcos-website