Fix: RTD Docutils Build (#3119)

The docutils 0.17+ release uses more semantic HTML5 tags, which
the RTD theme does not (yet) know.

This breaks side bar, lists and other elements.

docs/requirements.txt

 breathe==4.26.1
+# docutils 0.17 breaks HTML tags & RTD theme
+# https://github.com/sphinx-doc/sphinx/issues/9001
+docutils==0.16
 sphinx==3.3.1
 sphinx_rtd_theme==0.5.0
 sphinxcontrib-moderncmakedomain==3.17