Add JSON Form Widget readme

[rovcase]

Describe your changes

This adds a readme file to the JSON Form Widget module and updates related documentation accordingly.

QA Steps

• The documentation is accurate.
• Links within direct correctly.
• Images within the documentation display correctly and include correct alt text.

Checklist before requesting review

If any of these are left unchecked, please provide an explanation

• I have updated or added documentation

[dafeder]

A couple suggestions. Also wonder if we should more explicitly state that we have as a goal to make this module work independently of DKAN.

[rovcase]

Ty both! We'll chat a bit about where to add the diagrams and if/how to add the decoupling goal on Fridays huddle. Then one more review after and this should be good.