Add documentation

[dstansby]

• Add hosted API documentation #18
• Add docstrings across the package #68
• Document how NapariMPLWidget can be used by other plugins #24
• Add docs on how to access Matplotlib figure/axes once created #12
• Activate sphinx-gallery on doc build #106

[samcunliffe]

Only softly related to the docs, but if you like this comment enough to make a subtask: I'd put it here rather than elsewhere.

... As you're adding docstrings, is it worth adding type hints?

Could then add a mypy type check to the CI...? From my memory, this is quite easy with pre-commit.

A possible disadvantage is the change between 3.8 and 3.9 where the typing.List[] and other standard collections were deprecated. Unsure how annoying this would be.

[dstansby]

I'm 👍 to type hints - looks like I already added a bunch, but never added mypy to pre-commit. I'll open a new issue to track this.

[dstansby]

Don't think the final item will get done before this project finishes, so going to close this tracker issue as done.