Add technical documentation

[noisysocks]

I'm creating this issue to keep track of all of the technical documentation that needs to be written for the widgets editor which is landing in WordPress 5.6.

Non-technical documentation is generally handled separately by the docs team.

Please feel free to add to this list!

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For plugin and theme developers

• How to opt your theme out of block widgets.
• How to make it so that your widget doesn't appear in the Legacy Widget block dropdown.
• How to make it so that your plugin's widget can be converted into your plugin's block.
• How to add custom UI to the sidebar. See Widgets editor: Add basic options for extensibility #25758.
• How to add custom UI to the top and bottom of the screen. See Widgets editor: Add basic options for extensibility #25758.
• How to ensure compatibility with the Legacy Widget block using the widget-added event.
• How to show "No preview available." message in Legacy Widget block.

For core developers

• Overview of the screen's architecture. Should live in packages/edit-widgets/README.md.
• A dev note to go on Make/Core ahead of 5.8.

[annezazu]

Pulling from @justintadlock recent post on the new screen, I wanted to flag a few items he mentioned that aren't referenced here that seem great to address if possible:

• What are the hooks to add custom styles for the Widgets screen?
• Can themes target specific sidebar styles on the Widgets screen?
• Is it possible to consistently style sections like traditional widgets on the front end?
• Can themes opt into wide and full-alignment within block areas, which could essentially be used similarly to the post content area?

[skorasaurus]

For how to opt out #25123

[draganescu]

I am working on a dev note.