revamping the DEP index

It's done! Tomorrow morning I'll leave for VAC \o/.

As the last TODO bit I've completed before leaving there is a bunch of fixes for DEP (as in Debian Enhancement Proposals and their website:

Beside a bit of style, I've added content explaining briefly what DEPs are, synced the status of current DEPs with the reality, and added a state diagram of DEP0.

... and speaking about that, you'll soon hear on -project about DEP0 itself, though possibly not from zack-on-vac, yours truly.