feat: add readonly property to set whole form read-only

[Icemic]

Reasons for making this change

Just like existing disabled property, add readonly property to set whole form read-only.

Checklist

• I'm updating documentation
  • I've checked the rendering of the Markdown text I've added
• I'm adding or updating code
  • I've added and/or updated tests
  • I've updated docs if needed
• I'm adding a new feature
  • I've updated the playground with an example use of the feature

Deploy preview

Once CI finishes, you should be able to view a deploy preview of the playground from this PR at this link: https://deploy-preview-[PR_NUMBER]--rjsf.netlify.app/