Documentation: KUnit: Rework writing page to focus on writing tests

Documentation: KUnit: Rework writing page to focus on writing tests

We now have dedicated pages on running tests. Therefore refocus the
usage page on writing tests and add content from tips page and
information on other architectures.

Signed-off-by: Harinder Singh <sharinder@google.com>
Reviewed-by: Tim Bird <tim.bird@sony.com>
Reviewed-by: Brendan Higgins <brendanhiggins@google.com>
Link: https://lore.kernel.org/r/20211217044911.798817-6-sharinder@google.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>