Publish rustdocs to GitHub Pages

[daprilik]

We are already running cargo doc in our CI infrastructure, but we are not currently publishing those docs on GitHub pages.

This should be relatively straightforward to do in our existing pipelines, with the only nuanced bit being that we'd need to "splice" the HTML generated by rustdoc into the HTML generated by the mdbook-based OpenVMM Guide. e.g: having the Guide link to a _rustdoc/ folder, which the build system copies into the book/ folder prior to publishing to GitHub pages.

In addition, we may want to compile the rustdocs on both Windows and Linux (and possibly even x86 and Aarch64), and include multiple versions in the final HTML.