Add Guide and Example for theming functionality

We implemented the ability to add theming to widgets.

We should make sure that's properly encapsulated in https://developmentseed.org/manywidgets/

If it warrants it, we can create a new section for Theming, but also I think adding basic docs to the Guides/ section and then 2-3 examples going from simple to complex under Examples/ .

@dzole0311 do you think you can take this on? (Am otherwise happy to - I think I have a decent flow for creating new guides and examples).

Start by reviewing the existing Guides/ and Examples/ sections at https://developmentseed.org/manywidgets/ to follow the project’s guide and example structure. Document the theming functionality, adding a Theming section if appropriate, a basic guide, and 2–3 examples progressing from simple to complex.

Written by the indexing model from the issue text.