Add inheritance diagrams to the documentation

[robsdavis]

Add inheritance diagrams to the documentation. This was done previously with sphinx-pyreverse, but it ran into the following error, so has been removed for the time-being.

Previous Error

...
sphinx-pyreverse: Running: pyreverse --output png --project synthcity.metrics.eval_privacy synthcity.metrics.eval_privacy
reading sources... [ 13%] generated/synthcity.metrics.eval_sanity
sphinx-pyreverse: Running: pyreverse --output png --project synthcity.metrics.eval_sanity synthcity.metrics.eval_sanity
reading sources... [ 14%] generated/synthcity.metrics.eval_statistical
sphinx-pyreverse: Running: pyreverse --output png --project synthcity.metrics.eval_statistical synthcity.metrics.eval_statistical
Fatal Python error: Cannot recover from stack overflow.
...
  File "/home/docs/.asdf/installs/python/3.8.17/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['pyreverse', '--output', 'png', '--project', 'synthcity.metrics.eval_statistical', 'synthcity.metrics.eval_statistical']' died with <Signals.SIGABRT: 6>.

Exception occurred:
  File "/home/docs/.asdf/installs/python/3.8.17/lib/python3.8/subprocess.py", line 516, in run
    raise CalledProcessError(retcode, process.args,
subprocess.CalledProcessError: Command '['pyreverse', '--output', 'png', '--project', 'synthcity.metrics.eval_statistical', 'synthcity.metrics.eval_statistical']' died with <Signals.SIGABRT: 6>.
The full traceback has been saved in /tmp/sphinx-err-wxfpl1mt.log, if you want to report the issue to the developers.
Please also report this if it was a user error, so that a better error message can be provided next time.
A bug report can be filed in the tracker at <https://github.com/sphinx-doc/sphinx/issues>. Thanks!

This error can be re-created to look for a potential fix by adding "sphinx_pyreverse" to the extensions list in docs/conf.py.