Jupyter Book equivalent for empinken tags

[psychemedia]

Try to come up with some mapping fornb_extension_empinken tags. This isn't as simple as the tagstyler tags because we may have multiple contiguous empinkened cells that try to associate all the cells with a particular activity, for example.

Representing activities creates a whole other issue...: https://github.com/innovationOUtside/ou-jupyter-book-tools/issues/2

[psychemedia]

[Cribbed]

To pass a class tag for markdown content, the docs suggest that wrapping:

```{div} my-class
**Some content.**

will render:

Some content.

__But note that tags may get rewritten (`_` to `-`; see eg https://github.com/executablebooks/jupyter-book/issues/1365 )__

So for empinkened cell tags, we could have a cell preprocessor that:

- reads the tag from metadata;
- rewrites the cell content accordingly.