Update conf.py for current Sphinx

[prjemian]

The CI failure in #32 is because the conf.py file is out of date with current Sphinx. (need to comment the language= line now, the default is acceptable).

Originally posted by @prjemian in #32 (comment)

[prjemian]

The error:

make: Entering directory '/home/runner/work/area-detector-handlers/area-detector-handlers/docs'
Running Sphinx v5.2.3

Warning, treated as error:
Invalid configuration value found: 'language = None'. Update your configuration to a valid language code. Falling back to 'en' (English).
make: *** [Makefile:20: html] Error 2
make: Leaving directory '/home/runner/work/area-detector-handlers/area-detector-handlers/docs'
Error: Process completed with exit code 2.