Update block design principles with a new section on how to group con…

…trols

This takes a first stab at creating new documentation for block toolbar group best practices.

See also #25983.

docs/how-to-guides/designers/block-design.md

@@ -15,6 +15,12 @@ Since the block itself represents what will actually appear on the site, interac
 
 Basic block settings won’t always make sense in the context of the placeholder/content UI. As a secondary option, options that are critical to the functionality of a block can live in the block toolbar. The Block Toolbar is still highly contextual and visible on all screen sizes. One notable constraint with the Block Toolbar is that it is icon-based UI, so any controls that live in the Block Toolbar need to be ones that can effectively be communicated via an icon or icon group.
 
+### Group Block Toolbar controls with related items
+
+The Block Toolbar groups controls in segments, hierarchically. The first segment contains block controls, such as the block switcher, the drag handle, and the mover control. The second group commonly contains tools, followed by inline formatting, and the "More" menu. Optionally "Meta" or "Other" groups can separate some tools in their own segment.
+
+![A screenshot showing examples of block toolbar segment groupings.](https://make.wordpress.org/design/files/2021/03/docs_block-toolbar-structure.png)
+
 ### The Settings Sidebar should only be used for advanced, tertiary controls
 
 The Settings Sidebar is not visible by default on a small / mobile screen, and may also be collapsed in a desktop view. Therefore, it should not be relied on for anything that is necessary for the basic operation of the block. Pick good defaults, make important actions available in the block toolbar, and think of the Settings Sidebar as something that most users should not need to open.
@@ -27,7 +33,7 @@ Each Settings Sidebar comes with an "Advanced" section by default. This area hou
 
 Setup states, sometimes referred to as "placeholders", can be used to walk users through an initial process before showing the live preview state of the block. The setup process gathers information from the user that is needed to render the block. A block’s setup state is indicated with a grey background to provide clear differentiation for the user. Not all blocks have setup states — for example, the Paragraph block.
 
-![An example of a gallery block’s setup state on a grey background](https://make.wordpress.org/design/files/2018/12/gallery-setup.png)
+![An example of an image block’s setup state on a grey background](https://make.wordpress.org/design/files/2021/03/docs__gallery-setup-state.png)
 
 A setup state is **not** necessary if:
 
@@ -52,6 +58,12 @@ In most cases, a block’s setup state is only shown once and then further custo
 
 ## Do's and Don'ts
 
+### Block Toolbar
+
+Group toolbar controls in logical segments. Don't add a segment for each.
+
+![A screenshot comparing a block toolbar with good vs. bad toolbar segment groupings.](https://make.wordpress.org/design/files/2021/03/docs__block-toolbar-do-dont.png)
+
 ### Block Identification
 
 A block should have a straightforward, short name so users can easily find it in the block library. A block named "YouTube" is easy to find and understand. The same block, named "Embedded Video (YouTube)", would be less clear and harder to find in the block library.