Rewrite Phobos index page.

Alright. Since nobody else seems willing to do this, I'm gonna stick my neck out for this.

This PR completely rewrites the current Phobos index page to be more useful than the current outdated and irrelevant philosophy page. The old (completely outdated) list of imports is deleted, and I didn't find the philosophy section particularly accurate as far as the current state of Phobos is, so I deleted that as well. (I can put that back if people object, please speak up.)

I wanted to make this page maximally useful for first-timers, so I decided on an overview of the various Phobos modules available. Rather than stick to a single strict hierarchy, which is likely to be controversial, I approached it from more of a functional point of view: i.e., the user wants to know "I want to do X, does Phobos have a module for that?". I try to answer that by listing modules in broad categories that would help the user narrow down which modules he might be interested in. Since some modules can serve multiple purposes, I haven't been shy about putting the same module in multiple places. Not sure if this is the best approach, but at least it's a reasonable first stab at this IMO.

So. Destroy! :-P
P.S. I didn't find any page on wiki.dlang.org that seems appropriate; maybe we should get rid of that link completely? The current link seems to point to a page that's quite outdated and probably no longer reflects the current state of Phobos anyway.