Bug: Duplicated autoclass directive warning breaks doc build

[pushfoo]

Bug Report

tl;dr

1. A custom ClassDocumenter was added in cadf05e
2. Sphinx API doesn't provide a clear way to replace a doc builder rather than add it
3. The resulting duplicate autoclass directive warning is promoted to an error, breaking our build

System Info

arcade installed by branching off the tip of development (908664e)

Actual behavior:

Running doc build on CI or locally produces the following error:

WARNING: while setting up extension conf.py: directive 'autoclass' is already registered, it will be overridden

(See the doc build)

Expected behavior:

No error.

Steps to reproduce/example code:

Run CI, or install from tip of development and perform the following steps:

1. pip install -I -e .[dev]
2. ./make.py clean
3. ./make.py html