Redesign the sidebar for Gutenberg Handbook

[rilwis]

The sidebar of the Gutenberg Handbook is not user-friendly and should be updated:

​https://developer.wordpress.org/block-editor/tutorials/block-tutorial/writing-your-first-block-type/

Here are some problems with it:

• It's too narrow.
• The navigation has up to 3 levels, but the text of sub-levels has the same size and style as the parent, which makes it hard to scan.
• Dividers between items are not good enough, especially for sub-items.

I'd suggest:

• Make it wider, probably 300px or more
• Make the down arrow icon smaller or not taking too much space.
• Sub-items should be full-width, not having a large right padding like they do now.
• Decrease font-size for sub-items.
• Remove the dividing lines between items.

To make it easier to navigate, I think we can separate items into categories (top-level item = category, 2nd-level = items in the category), put each category title bolder, maybe uppercase. Separate them with space, like this:

​https://i.imgur.com/hiKsLm8.png

[coffee2code]

In 9428:

Developer theme: More handbook sidebar menu design improvements.

• Decrease padding-right for menu items which do not have children
• Embolden the name of the current page in the menu

Props rilwis, coffee2code.
See #4901, #4467.

[coffee2code]

In 9429:

Developer theme: Decrease width of sidebar menu expand/collapse buttons.

Also fixes emboldening of active menu item.

Props rilwis, coffee2code.
See #4467.

[coffee2code]

In 9430:

Breathe theme: Improve handbook sidebar menu design.

• Eliminate per-item top/bottom borders
• Use WCAG-compliant color (as used for links) for left border
• Increase thickness of left border for menu sub-items
• Reduce padding-left to align widget title with leftmost text within widget
• Decrease padding-right for menu items which do not have subchildren
• Embolden the name of the current page in the menu

Props rilwis, felipeloureirosantos, dufresnesteven, coffee2code.
Fixes #4901, #4467.

[coffee2code]

In 9431:

Breathe theme: Decrease width of sidebar menu expand/collapse buttons.

Props rilwis, coffee2code.
See #4467.

[coffee2code]

#4901 is a related ticket which has led to a number of improvements to handbook sidebar menus. Between changes made for that ticket, and changes referenced above, I consider this particular request fulfilled.

Of the five suggestions made in the ticket, only two have not been implemented:

• Make it wider, probably 300px or more
• Decrease font-size for sub-items

At this time these particular changes don't seem warranted.