Feature: custom/vanity domain for docs.rs

[rsaffi]

Hi, I've asked on Discord about the possibility of using a custom/vanity domain for docs hosted on docs.rs, something like what ReadTheDocs allows for, so instead of having docs.rs/some-project, having that reachable via docs.my-project.org, for instance.

As per @pietroalbini's suggestion, I'm opening this issue to register the idea and keep track of it! :-)

Thanks in advance and keep up the good work! :-)

[syphar]

this seems to be a duplicate of #1853