Use the more powerful configuration file .readthedocs.yaml to ensure
that all requirements are installed. This now includes also a distro
package, python-newt, something that cannot be installed via a
requirements.txt.
Fixes the doc build for 2.6.x which become broken with the addition of
the menu plugin.
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>
