Assessment action missing from AP model

[pkothare]

The content on https://pages.nist.gov/OSCAL/resources/concepts/layer/assessment/assessment-plan/ states that the assessment plan should contain a field for assessment actions, but I couldn't find it in the JSON/XML model, nor any references to it in the metaschema. Based on comments by @iMichaela in usnistgov/OSCAL#2020, the diagram and text need to be updated, and not the model itself. For reference, this is what I observed:

The file that needs to be updated is: https://github.com/usnistgov/OSCAL-Pages/blob/develop/src/content/resources/concepts/layer/assessment/assessment-plan/_index.md