Improve the navigation around Handbook pages, especially on mobile

[hedgefield]

I spend some time in the design team handbook, and I find the navigation and general structure of it a bit frustrating.

The top-level items (or landing pages) all tend to be thin on content, and when viewing them on mobile you don't see the sidebar menu with the TOC, so you're left to wonder "...is this it?". With the prev/next buttons at the bottom you can navigate to the next page, but it's not clear those are related, it feels like you'll be moving on to the next topic.

Then once on one of those subpages, the navigation buttons at the bottom are gone, so unless you discover the hamburger menu, you're trapped.

I'm not exactly sure how yet, but these pages need a better flow, more cohesion.

[hedgefield]

Some related tickets: #425, #2871

[coffee2code]

In 7723:

Handbook plugin: Add greater specificity to cache key to prevent cached posts from being used in the wrong context.

Fixes bug where navigation links often didn't appear.

See #3757.

[coffee2code]

Closing this as fixed after [7723] fixed the page-to-page navigation links and with the introduction of breadcrumbs via #3965.

Now, for narrow viewports, at least as far as navigation goes, we have:

• Above the content, breadcrumbs leading from the current page up through each ancestor to the main handbook page and homepage.
• Below the content, navigation links to the previous and next pages according the order defined by the sidebar menu.
• As a slide-out menu accessible via a hamburger icon, the full chapter listing of all pages in the handbook.

Feedback about the content of the pages (or lack thereof) or how they are organized are best directed towards the team in question, who is ultimately responsible for their own handbook. I'm sure they'd appreciate help in fleshing out the content.