fix(plus): add breadcrumbs for Collections/Notifications/Settings

[caugner]

Summary

Adds breadcrumbs for Collections/Notifications/Settings in MDN Plus.

Problem

MDN Plus did not have breadcrumbs (except for the "Documentation" section).

Solution

Make it easy to add breadcrumbs to non-Doc pages.

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Screenshots

Before

After

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How did you test this change?

Checked out these pages locally:

• http://localhost:3000/en-US/plus/collections
• http://localhost:3000/en-US/plus/notifications
• http://localhost:3000/en-US/plus/settings

[github-actions[bot]]

This pull request has merge conflicts that must be resolved before it can be merged.

[Guyzeroth]

1 change then LGTM. Thanks for picking that up 👍

[Guyzeroth]

Coolio. Thanks! :)