doc: update quick start guide for meson

doc: update quick start guide for meson

The build-sdk-meson.txt file is a little out of date, so update it with
information on the latest build requirements, and remove any content
no longer needed.

Since the cross-compilation file quoted in the document is now considerably
longer and more complex than previous, replace the contents of the file
with a summary of it instead. This is shorter and more maintainable, and
the original file is available as part of the repo anyway if the user wants
to view it.

Signed-off-by: Bruce Richardson <bruce.richardson@intel.com>