Add a documentation index to the generated docs

[brson]

Right now our docs link on the website leads to the wiki, which then leads back to the generated docs. Make this page generated as part of the documentation build process.

[alan-andrade]

On it.

[alan-andrade]

Is this resolved? I forgot I stepped up for this one, but I just figured out that is related to the wikis PR.

[alexcrichton]

Closing, this now exists: http://static.rust-lang.org/doc/master/index.html