Documentation: add description of plugin support in the edit-site package

[t-hamano]

What problem does this address?

With #34460, the following basic plugin support has been added to the Site Editor:

• PluginSidebar
• PluginSidebarMoreMenuItem
• PluginMoreMenuItem
• openGeneralSidebar / closeGeneralSidebar store actions

If we use these with plugins, we need to use them differently depending on whether the post editor or the site editor, as follows:

// In the post editor
import { PluginSidebar, PluginSidebarMoreMenuItem } from '@wordpress/edit-post';
// In the site editor
import { PluginSidebar, PluginSidebarMoreMenuItem } from '@wordpress/edit-site';

However, it is not explicitly mentioned in the tutorial in the Block Editor Handbook, which may confuse developers, as seen in #38547.

What is your proposed solution?

I suggest the following documentation updates:

Add API description

• packages/edit-site/README.md

Specify that the source package is different in the site editor

• packages/plugins/README.md
• docs/reference-guides/slotfills/plugin-sidebar.md
• docs/reference-guides/slotfills/plugin-more-menu-item.md
• docs/reference-guides/slotfills/plugin-sidebar-more-menu-item.md

[t-hamano]

Closed for reasons stated in this comment.