docs: Fix broken link to environment setup

[Spiteless]

Update docs link to https://rouge-ruby.github.io/docs/file.DevEnvironment.html

[tancnle]

Thanks for your contribution @Spiteless ❤️

FYI, we are using YARD to generate the documentation on the Rouge docs site. Ideally, it would be better the let the tool handle the parsing of the URLs in the markdown file, ie. using Yard Server. Unfortunately, our doc site is hosted in GitHub pages as a static site and hence these references must be absolute. I don't know if there is a better solution than your proposed changes so let's ship this 🚀

[Spiteless]

I hear that. I looked elsewhere in this codebase and this was how it was done too.