Icon for the generated doc

[daladim]

This adds the Diesel icon on the pages generated by cargo doc.

That's a draft however, because:

• We may prefer an image that does not include the white "Diesel" text (especially to play nice with the light theme). However, I'm not sure such an image already exists. I could create it, but I'm not sure yet if it would be accepted.
• I'm not sure the URL I'm pointing to is guaranteed to be always valid in the future. Is it? Should we include e.g. a version number in its URL?

[daladim]

Here is what the rendered doc now would look like.
(the cannister alone, without the text, would be better aligned imho)

[weiznich]

I think it's fine to assume that the logo stays on the web page for some while. If someone submitts a PR there, we can also host only the logo without the text.