Create "How to" guide in block editor handbook for moving away from legacy metaboxes

[annezazu]

As part of a PR to add a toggle for metaboxes, the topic came up around how we can improve messaging and resources for folks to use the new tools we have instead of relaying on legacy options. As part of this, I think it would go a long way to have an explicit doc around this, pulling together various APIs and sharing a range of examples. Opening this issue so the idea isn't lost.

[youknowriad]

There are multiple possible answers here:

• Plugin Sidebar
• Document Panel in the "post/page" panel
• Blocks bound to custom fields
• ...

The first one has been used already by several plugins and already has a guide for it https://developer.wordpress.org/block-editor/how-to-guides/plugin-sidebar-0/