Update sphinx_rtd_theme and docutils

• build: https://readthedocs.org/projects/bind9/builds/22913803/
• preview: https://bind9.readthedocs.io/en/tkrizek-update-sphinx_rtd_theme/

No visible changes, looks ok.

Apparently, we lose support for building docs on RHEL 8 and older, but our note in the file says it's fine...