Link to docs for changing the build environment in issue templates

[jyn514]

Addresses #504

The motivation here is to reduce the maintenance burder on the docs.rs team by having people submit PRs to https://github.com/rust-lang/crates-build-env directly. This has 2 main benefits:

1. Crate authors already know the packages they need, so they don't have to tell us and have us make a PR ourselves; this removes an intermediate step.
2. If crate authors are wrong about the packages they need (for instance, No Fortran compiler available #483 needed liblapack-dev, not just gfortran), they get immediate feedback instead of having to wait for a deploy and republish their package.

[jyn514]

Note that this only changes the issue templates, not the links on a failed page.

[jyn514]

This is ready for review.

[pietroalbini]

Thanks!