[DO NOT MERGE - NEEDS REBASE] fix bug 1085623 - reorg Socorro docs

r? anybody - I know @lonnen is interested, maybe if @erikrose could take a look (it's inspired by DXR's excellent docs)
@phrawzty re: your suggested text... see previous comment about where that *should* be linking to, but I think giving people pointers to build from source is reasonable.

However right now Vagrant is treated just like the other platforms, so I think it'll be more confusing if we call it out separately... the point is that you can install any "platform" and then do an RPM install *or* you can build from source. Does that make sense or should we split out Vagrant somehow?