getting_started: building: fix inaccuracies

The building and running section is out of date, as Windows and macOS
commands are available now.

It also has some accuracies:

- the following section describes how to build hello_world, not how to
  make a new application based on it

- zephyr-env scripts do more than just set project-specific
  environment variables (they modify PATH on Unix, and may run
  arbitrary RC scripts on all platforms)

Fix these issues.

Signed-off-by: Marti Bolivar <marti@opensourcefoundries.com>