Add root-path

[GuillaumeGomez]

r? @QuietMisdreavus

cc @onur

I tested everything that came to my mind but I'm pretty sure I didn't think of a corner case where this feature could break. Any feedback/additional tests would be very appreciated. :)

[onur]

Hi @GuillaumeGomez, this is great, I haven't tested it yet but there are some issues:

1. This won't allow me to add a version subfix to commonly used files. It's still not compatible with theme picker. JavaScript portion of theme picker is only working with dark.css and rustdoc.css. That's the main reason why theme picker did break docs.rs. I have to add a rustc version to this files in order to keep compatibility between docs generated with the old versions of rustc.
2. search-index.js is usually different for each create, I am not moving it in docs.rs and keeping it in doc/ directory of every crate.

I'll test it as soon as I find some time and return to you if I find anymore issue.

Your --url or --replace idea is actually exactly what I am looking for instead of generating this files in another location. But thanks to you I know where to look at for this feature now, maybe I can help with a solution.

[GuillaumeGomez]

Ok, writing a second PR which adds an resource-suffic option.

[shepmaster]

Ping from triage, @QuietMisdreavus !

[GuillaumeGomez]

I'll close this PR in favor of #48511.