Add logo to docs

[jturner314]

I noticed the logo in the upper left corner of the wgpu docs and thought it was kind of fun, so I've created this PR. This is the same logo I created for the rust-ndarray GitHub organization. The Ferris crab inside of the brackets is from here; it's licensed as CC0 (public domain).

Note that the image URL will not work until this is merged (since the image will be hosted by GitHub). Edit: I just saw that a PNG version of the logo is currently located in the GitHub pages repository. Would it be better to host the image from GitHub Pages instead?

[bluss]

I don't know if github pages or githubusercontent is the best one, depends on what github thinks about serving files out of the latter? They probably tolerate a lot of that, but there are solutions like https://gitcdn.link/ for that too (so it must mean there is some drawback like throttling? https://stackoverflow.com/questions/8779197/how-to-link-files-directly-from-github-raw-github-com

Use pages if you want. It should only be hosting an ancient version of the rustdoc (that's redirecting to docs.rs) (no need to change anything about the redirect I guess)

[bluss]

This is a good solution, but we can use pages too if you want

[jturner314]

I've uploaded the logo to the rust-ndarray/rust-ndarray.github.io repository.

I did notice that the only other image in the docs (the one for .axis_iter()) is hosted from the gh-pages GitHub Pages branch on this repository rather than the separate repository, but there's also a similar image in the separate repository. I'm not sure what the intent is with respect to the gh-pages branch here vs. the separate repository.