readthedocs: Add configuration file

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>

docs/requirements.txt

@@ -0,0 +1,3 @@
+sphinx>=1.2.0
+CommonMark>=0.5.6
+sphinx-argparse>=0.2.1