Fleshed out front page

I've added text to everything except Downloads (not clear what should go there, since you already have a big, green "Download" button at the top of the page). I've commented out part of the text for Mechanics->Connections, and the entire "List of parts" section, on the grounds that that material should be moved to detail pages. I've also reworded some of your text to prevent possible confusion for new players.

I'm in the process of going through the forum thread to identify what seem to be the most common misconceptions about how RT works, but there's a lot to wade through. I figured I shouldn't delay the review just because I might add a sentence or two later.