Add link to rust documentation

[anaran]

This is my prototype for addressing issue #117.
I have not added any css rules, which would probably be necessary for the Documentation link to be visually pleasing.
Also, the destination of the link might as well go to the rust book instead.
Anyway, have a look at https://anaran.github.io/rust-playpen/static/web.html for what this pull request currently does.
If there's interest I would be happy to make adjustments and squash this into something that can be merged.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @alexcrichton (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. The way Github handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see CONTRIBUTING.md for more information.

[alexcrichton]

Thanks for the PR! I think we may want to touch up the style of the link to be consistent with the rest of the look-and-feel as well though.

cc @chris-morgan

[anaran]

Right. I didn't see any css rules I should obviously reuse for that link.

Also, perhaps the link should directly point to
https://doc.rust-lang.org/book/hello-world.html
which would be more contextual for new visitors.

It might even inspire them to install rust on their machine as they want to do more, like using crates.

[alexcrichton]

Either redirecting to the main site or just /book I think is fine, those are both good starting resources at least!

[chris-morgan]

I really don’t think that such a link belongs on the header bar. I have in mind keeping it hidden inside an information screen kind of like the Go Playground’s “About” button (I have in mind a ℹ button next to the configuration button). Now adding a dropdown with code examples, that’s something that would belong in the header bar.

[anaran]

@chris-morgan These are great ideas. I guess it wouldn't be very efficient for either one of us if I tried to implement what you have in mind exactly.

[chris-morgan]

@anaran You could be right in principle, but I do not intend to implement it in the very near future; I’m working on other things. Up to you, really.