Add a wtperf page to the documentation.

@agorrod, @michaelcahill -- I took a run at adding a wtperf page to the documentation, I thought it might be useful for people wanting to simulate workloads (and maybe we'll have to debug fewer homegrown specials?)

I included a list of wtperf configuration options in the page (output by the new -Q option to wtperf). I don't see any easy way to automatically build that list, since wtperf might not be built when we're building the documentation, so that's probably a new step in the release-generation process.

I also did some minor surgery to the front-page of the documentation, splitting up the admin and programming  information.

I thought the next step might be to move some of the statistics logging/visualization information to the same page (https://github.com/wiredtiger/wiredtiger/wiki/WiredTiger-statistics), giving programmers a way to simulate workloads and actually visualize what's happening?

Anyway, if you think this is a good idea, take it, if this isn't worth the effort or will buy us more trouble than it's worth, I'm happy to discard it.