Reorg installfest

Paging @janetriley - sorry for delay! I started this after the last workshop but wanted to clean it up.

This is a big reorganization, and will need editing after it lands - I will try to find as many OSes as possible to test on, and make the per-OS parts more specific.

Main change is the new downloads page, which uses a table. Students find the column for their OS, and download each file in the column. I got positive feedback on this in a real-world "hey, look at this" test. Hopefully it feels more streamlined to match the streamlined purpose (just download the files). Made the "re-download in case versions changed" bigger, because they'll always change (I'll update versions after this lands).

The installfest page splits out the per-OS instructions into separate pages -- we sort of did that before, but some VM instructions were mixed in with some OS instructions. Goal here is to only cover how you run an installer on the particular OS, and then merge as much of the VM setup steps as possible into the VM setup page, which is the next step on the checklist. These steps need some tuning! For example we can be specific with filenames instead of ".exe or .msi" But this is just step 1, which is applying some organization.

(one thing there is that we may want to move back the creation of `~/workspace` to the OS pages, if we have to.)

Instructions themselves: I've tried to give the DIY Linux instructions some love, and added a new platform, Arch. They need real-world testing, though. OS X Xcode downloads are updated as well. I've added a hint that you can also do these as a sort of "expert mode", and if any students show interest, I could lead a breakout session on how to install "real" Ruby after you complete everything as a way to refine those steps.

One final thing: to do tables (like with alert boxes), you have to drop down to HTML. This led me away from doing anything fancy where prose was in the table cells. However, if in the future someone wants to add a JS library that makes the per-OS instructions into a tab sheet, that would be great!

Any other suggestions (from anyone out there) are very welcome. I hope that this is a step toward making future tweaks (which tend to be OS-specific) easier for volunteers to do and us to merge. I am busy until Friday but can follow up with feedback after that.
Pushed an update to this link that should take you to an Apple ID login page if you're not logged in yet.