doc: application: Document west outside workspace

Document that using west outside of a west workspace needs
ZEPHYR_BASE to be set. If this is not set, the instructions
on the "Application Development" page don't work.

Previously, "Getting Started Guide" and "Application Development"
pages told you to run `zephyr-env.sh/zephyr-env.cmd` which would
set ZEPHYR_BASE, but this was removed in Zephyr 2.3.0

Signed-off-by: Daniel Wong <drojfjord@gmail.com>