ipyleafelt's `add_*` methods were deprecated: update examples

[banesullivan]

See https://github.com/jupyter-widgets/ipyleaflet/pull/982

The add_* methods like add_layer were deprecated. The examples for ipyleaflet need to be updated accordingly.

I may give it some time for the latest release to settle and for users to start getting the deprecation notice before making these updates to minimize friction for new users