docs: Add Volto 19 documentation for add-on development

[Shyam-Raghuwanshi]

Add Volto 19 documentation for add-on development

• I signed and returned the Plone Contributor Agreement, and received and accepted an invitation to join a team in the Plone GitHub organization.
• I verified there aren't other open pull requests for the same change.
• I followed the guidelines in Contributing to Volto.
• I successfully ran code linting checks on my changes locally.
• I successfully ran unit tests on my changes locally.
• I successfully ran acceptance tests on my changes locally.
• If needed, I added new tests for my changes.
• If needed, I added documentation for my changes, either in the Storybook or narrative documentation.
• I included a change log entry in my commits.

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If your pull request closes an open issue, include the exact text below, immediately followed by the issue number. When your pull request gets merged, then that issue will close automatically.

Closes #7633

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If your pull request includes changes to the documentation—either in narrative documentation, Storybook, or configuration—then a pull request preview will be generated and a link will populate in the description of your pull request below.
By clicking that link, you can use the visual diff menu in the upper right corner to navigate to pages that have changes, then display the diff by checking the Show diff checkbox.

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📚 Documentation preview 📚: https://volto--7694.org.readthedocs.build/

[stevepiercy]

Please read the directions in the original issue.

[stevepiercy]

I'm sorry, but I've lost faith that you actually ran through this process for Volto 19.

Can you explain what are the differences between Volto 18 and 19 for these processes?

[stevepiercy]

Thank you! This looks a lot better. A couple more minor suggestions, then a final technical review, and it can be merged.

[stevepiercy]

This looks good from an editorial perspective.

We need one technical review and approval, then it's OK to merge.

Thank you!

[davisagli]

LGTM