Improve the Mockup documentation

[thet]

This is a preparation to include the Mockup documentation in the Plone documentation.

[stevepiercy]

This is great. I made some minor grammar and MyST syntax changes.

[petschki]

LGTM!

[petschki]

I'd say lets publish this state to the documentation. We can fix the gaps afterwards.

[stevepiercy]

There are two unresolved questions. Once those are answered and fixed, then I approve and we can merge this. Please let me know. Thank you! @petschki @thet

[stevepiercy]

I will merge if it goes green. Thank you!