docs: Upgrade to Sphinx 3

Still needs an update of the CI container before it can be merged. Hawkmoth support for Sphinx 3 is working well, the only bigger issue was a problem in Sphinx itself, see https://github.com/sphinx-doc/sphinx/issues/8945 and commit f9f33dc9 ("hack(docs): Add workaround for Sphinx issue #8945").

@q3k, I updated the Dockerfile, can you rebuild and push that to your registry please?

Documentation/bluetooth/ess.rst

@@ -17,7 +17,7 @@ If :ref:`bsec_api` is enabled the following additional estimates are available:
  - Indoor air quality (IAQ estimate
  - Equivalent CO2 (eCO2) estimate
 
-Please refer to :ref:`bme680` for more information about BSEC.
+Please refer to :py:mod:`bme680` for more information about BSEC.
 
 
 If notifcations are enabled a measurement of all values is performed every 3 seconds. For each measurement a notification is sent for the characteristics which have notifications enabled.