Update index.php

Changed introduction to highlight forum. (Since it has the most activity.)
Made paths for nav, css, and js relative to root

Just hope it works the same on server as locally...
Added prefix to relative paths

with a fixer function so they work when running on localhost
Merge pull request #1 from FlyingHyrax/local-paths

Made CSS, JS, and nav.json paths relative, so we can use PHP's local server for testing
Merge pull request #2 from internetwork/patch-1

Update homepage w/ forum link