Adds docs for creating custom control panel #1909 revision

[stevepiercy]

@rohnsha0 please take a look at these revisions. I haven't been able to express myself clearly in a review. I think this PR simplifies its structure and makes it flow better. Please let me know. Thank you!

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📚 Documentation preview 📚:

• https://plone6--1939.org.readthedocs.build/developer-guide/create-backend-add-on.html
• https://plone6--1939.org.readthedocs.build/developer-guide/create-control-panel.html