Plugins page in docs

[kaitj]

The problem

Raised initially in #291 - a new page should be added in addition to the API And Interals page for plugins on the docs.
Will need to check if the docstrings in the dunder methods (e.g. __call__) are automatically documented.

Steps to reproduce

N/A

Screenshots

N/A

Environment

• snakebids version (if applicable): N/A
• Operating system (e.g. Ubuntu 20.04, Windows 10): N/A