Add breadcrumbs to handbook pages

[coffee2code]

When on a given handbook page, it can be difficult to determine the context of that page within the hierarchy of the handbook. If the sidebar is visible, a visitor can use that to ascertain the context, though perhaps not always quickly or clearly if the sidebar menu is sizable. At narrow viewports where the sidebar gets hidden under a hamburger icon, there is no indicator of context.

Breadcrumbs would make a handbook page's context clear, and facilitate navigating up to one of its ancestor pages.

Note: The handbooks in DevHub have already long had breadcrumbs. Adding support to the Handbook plugin will make the feature more universally available and should eventually supplant DevHub's custom version.

Attached is a screenshot showing how a handbook page would look at a narrow viewport with no sidebar but with breadcrumbs. Without the breadcrumbs, it's not immediately apparent whereabouts within the handbook the page is.

[coffee2code]

Mockup of breadcrumbs on narrow viewport, as mentioned in ticket.

[coffee2code]

In 7916:

Handbook plugin: Add support for breadcrumbs.

See #3965.

[coffee2code]

In 7917:

Breathe: Show breadcrumbs for handbook pages.

See #3965.

[coffee2code]

In 8047:

Gutenberg: Show breadcrumbs for handbook pages.

See #3965.

[coffee2code]

Resolving this as fixed.

All the Make handbooks, and now the Gutenberg handbook, have breadcrumbs. In the ticket I mentioned converting DevHub's existing breadcrumbs to use the implementation provided by the Handbooks plugin, but the DevHub implementation is more robust and used elsewhere on the site (e.g. the Code Reference) so it can't be so simply replaced.