Minor modification of documentation build to get rid of warnings from inherited docstrings

[rossbar]

What

• Adds sphinx.ext.autosummary and use the .. autosummary directed to auto-generate the docs for the custom layers.

Why

• The default level of introspection prevents diving down into inherited members, where issues in the parent docstrings (from tensorflow) throw sphinx warnings/errors in this doc build.

Note - this is not the only solution for fixing this issue, but IMO is a relatively lightweight one that aligns with best-practices for reference docs. Note that this will turn the deepcell.layers page into more of a summary listing; though users can still click through to see the ref docs for each class.

[rossbar]

This rabbit hole is a lot deeper than I expected. Closing in favor of alternative approach...