Update `context_expressions.md` now that expressions moved to `app-model`

[lucyleeow]

Expression implementation has been moved to app-model (which has some okay docs: https://app-model.readthedocs.io/en/latest/reference/expressions/#app_model.expressions.Expr)

Also app-model does not yet handle contexts but we are using their context types (e.g., ContextNamespace and ContextKey).

We should update the documentation to reflect this and point to app-model docs where appropriate.

Related ref: https://github.com/napari/napari/issues/4860#issuecomment-1586451648

[lucyleeow]

Related task #146

[lucyleeow]

We should also include the new ContextMapping, in particular:

• allows function context keys
• new instance created every time we get_context
• context keys can behave in one of two ways:
  • context keys connected to events are updated immediately, and will always have the 'correct' value
  • function context keys only get evaluated when we get_context, this is why we create a new context instance and update from context on aboutToShow.

See for more: https://github.com/napari/napari/pull/6965#discussion_r1650358495

Adding this here to group context updates together.