Write a tutorial for adding a new parser & calendar converter

[rlskoeser]

Maybe this can be done as part of implementing support for the French Republican calendar (#136).

We have API level documentation for the converter class, but it would be helpful to provide higher-level documentation about the whole process, from writing a lark parser and transformer, to implementing the converter class and defining the calendar, testing, and ideally demonstrating/testing functionality in a notebook.

This documentation may need to be updated regularly, since development on other functionality (like uncertain duration) continues to have an impact on the converter classes as we identify more and more details that should be calendar-specific.