There should be documentation for Executability

[gerrycampion]

1. The rule schema should be updated to include descriptions for Executability and each of the Executability options. This should work with intellisense.
2. The rule editor author guide should be updated to include a section for Executability.

[SFJohnson24]

note: the second point is updated here: https://github.com/cdisc-org/conformance-rules-editor/blob/main/docs/_sidebar.md you will need to create a new markdown file and add it to the sidebar which is how the https://cdisc-org.github.io/conformance-rules-editor/#/ reference guide is constructed