fix interpret-community sphinx docs build failures by specifying html_theme

[imatiach-msft]

The interpret-community docs build started failing with errors because html_theme has become mandatory (I'm not exactly sure why, since sphinx is pinned - perhaps another package it depends on was updated?):

Traceback (most recent call last):
  File "/home/docs/checkouts/readthedocs.org/user_builds/interpret-community/envs/v0.30.0/lib/python3.8/site-packages/sphinx/config.py", line 329, in eval_config_file
    exec(code, namespace)
  File "/home/docs/checkouts/readthedocs.org/user_builds/interpret-community/checkouts/v0.30.0/python/docs/conf.py", line 152, in <module>
    'html_theme': html_theme,
NameError: name 'html_theme' is not defined

Specifying the html_theme in this PR to resolve the sphinx doc build errors.

Also updating sphinx package to latest.

Link to passing build:
https://readthedocs.org/projects/interpret-community/builds/21721970/