Edge cases with executing all notebooks

`just execute` executes all .ipynb files in the `notebooks` directory.

This can cause issues if there are errors in execution of notebooks not relevant to the user's notebook(s) they're working on (e.g., in a PR).

## Solutions

One of:
1. `just execute` only executes a single or list of notebooks passed as arguments (e.g., `just execute notebooks/my-notebook.ipynb`
2. `just execute` executes only unexecuted notebooks.

Option one is probably better and less complex, putting the onus on the authoring user.

_Flagged in https://github.com/developmentseed/manywidgets-playground/pull/4#issuecomment-5045464615_