doc: ci: exclude unneeded files from documentation tarball

There is no point in including source .rst files in html-output.tar.xz
as it unnecessarily slows down the creation of html-output.tar.xz as well
as transfer to/decompression on S3. Same for Doxygen XML output which is
only useful at build time.

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