readthedocs: install missing dependency sphinx_rtd_theme

According to: https://sphinx-rtd-theme.readthedocs.io/en/stable/installing.html
the dependency should be added to requirements.txt.

It was working previously becuase it was automatically installed by
readthedocs, this has been changed: https://blog.readthedocs.com/python-core-requirements-changed/

doc/requirements.txt

@@ -1,2 +1,3 @@
 breathe
 sphinx-autoapi
+sphinx_rtd_theme