Hooks env variables documentation

[MarkoMin]

There are environment variables that get exported for running hooks (link), but I struggle to find them in the documentation. I had this issue which was resolved by using one of those variables and IMO it would make sense to document those variables in "Hooks" section of rebar3 docs.

I could come with a PR, if anyone can confirm that:

1. they're not already documented somewhere (I tried to find them, but with no success)
2. it would make sense to document these

[ferd]

The rebar_env module is the canonical source of variables, yeah. It appears we never clarified all the variables in the docsite, but we probably should add them.

Part of the challenge will be also that rebar_dir isn't really documented either, and maybe it would eventually fit under the plugin documentation but we can ignore that extension to docs for now.