Extract docs about supported environment variables into its own help page

[mislav]

Right now, environment variables section takes a lot of space in gh help output.

Idea: gh help env (or man gh-env on non-Windows systems) could display a separate help page for detailed information. This page could be referenced from other help pages.

Right now, we only have support for generating help entries and man pages from runnable Cobra commands, but maybe we can add some support for extra help pages that are not tied to any command.

Ref. #1370

[vilmibm]

my current plan is to add an env command that reports on all the environment variables we care about as it could help for debugging; then we get ghd help env for free.