doc: unify documentation on setting environment variables

There are a few different places where alternatives for setting
environment variables are described. None of them is 100% complete, so
the results are likely to be confusing.

Make a single page on setting environment variables, how the zephyrrc
files work, how the zephyr-env scripts work, and some of the important
environment variables, with appropriate references elsewhere. (This is
inspired by the Arch wiki's excellent page on installing programs.)

Link to it from the getting started and application development pages
instead of repeating the information. This has the benefit of
shortening the getting started guide a bit more.

Add some concrete advice on checking the toolchain environment
variables in particular. This is a stumbling block for beginners.

Signed-off-by: Marti Bolivar <marti.bolivar@nordicsemi.no>