Reorganize documentation index

[brson]

See http://doc.rust-lang.org/

This page probably presents the worst navigation in the new user experience right now. There's a lot on it and it isn't obvious what's important. It's also missing some key community resources that basically all new users should read (rustbyexample.com and rustforrubyists.com).

Some ideas:

• Most of the crates we distribute are not that interesting, particularly for new users. How do we explain which ones they should pay attention to? Maybe some categorization would help.
• rustbyexample.com and rustforrubyists.com are both too good not to direct people to, but they are both maintained externally and not distributed with our (versioned) documentation. How do we incorporate these?
• This page doesn't explain how to get help, though it does link to the right places.

[steveklabnik]

Agreed, on all counts. I'll start to give this some thought.