doc: twister: inline `twister --help` output into the docs

In order to make it easier for folks to discover available twister
options when using/searching the documentation, or when making external
search in their favorite search engine, inline the output of
`twister --help` into the docs so as to have an always up-to-date
and searchable snapshot of it.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>