docs: Pin sphinx-rtd-theme version fixing build error

Previously the 0.4.3 version was correctly used. There are still some
warnings during build, but those were also present previously (verified
by checking old RTD build).

requirements-docs.txt

@@ -2,6 +2,6 @@
 
 CommonMark==0.7.5
 sphinx>=1.5,<1.6
-sphinx_rtd_theme
+sphinx_rtd_theme==0.4.3
 sphinx-autobuild
 git+https://github.com/leapp-to/recommonmark.git@master