feat(Breadcrumb): Update behavior guidelines

What does this MR do?

With updates to the navigation, the breadcrumb documentation has fallen behind as the source of truth. This change aims to bring guidance back in line with the expectations while also defining the behavior and content necessary to support the constraints in which the breadcrumb appears.

Related #1562 (closed)

Unblocks gitlab-org/gitlab#358113 (closed)

Illustrative example

These mockups are not a perfect example, but they are meant to show the constraints the breadcrumb is forced into.

Before	After

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