build: UI docs and add plugin_builder.py

[jnumainville]

Fixes #627

Also adds a new tool plugin_builder.py that can be used to make some tasks like installing and developing plugins easier. The basic principle is you can either target specific plugins or target all plugins, depending on the command(s). This is a basic version and we can add on capabilities. Maybe unit tests or something that emulates editable installs for example. Specifically in this case, python tools/plugin_builder.py --docs ui can help test (although the requirements need to be installed first still).

[jnumainville]

@mofojed one thing I'm struggling with is the fact that we still have the plotly plugin around. It conflicts with the plotly-express plugin, which is not great when the default behavior is to install all plugins, including both of those. I played around with some sort of deprecation object, but it would make some of the implementation uglier for a package we won't be developing anymore anyways. Thoughts? A warning in the readme? Deleting the plugin? Moving it to a deprecated folder within plugins?

[mofojed]

Keep a README to notify of deprecation, delete the rest of the plugin.