Make openDocs more flexible

[dmyyy]

openDocs is really useful - unfortunately the docs aren't always hosted on docs.rs. It would be nice to be able to

• open locally built docs
• custom urls - i.e. docs for Bevy's next release are hosted here. It would be nice to be able to point there when working with Bevy main

[Veykril]

If you have built the docs locally I think we should already be opening the local ones (with VSCode at least).