`--base-url` option for rust-book link?

[manmen-mi]

well... I'm not good at English, so I've checked rust-book link pretty many times from terminal hints.
but, those link navigate to official documents(https://doc.rust-lang.org/book/chXX-XXX), and I copy - change domain - paste every time after open links.

maybe rustlings provide options for change url domain, it would be really really helpful for non-bilingual peoples.
_{(this description has been helped by google translate)}

[manmen-mi]

@mo8it hi there, I made PR (#2172) for this issue. May I request to review it?
(Sorry if you are reviewing)