Nested sidebar nav for certification and Front-End Sites section

[stevector]

This pull requests starts integrated a nested sidebar element. (I'll capture a fuller plan under #9084)

To test the working nested sidebar, go to

• https://pr-9047-documentation.appa.pantheon.site/certification/study-guide
• https://pr-9047-documentation.appa.pantheon.site/guides/decoupled/overview
• And any guide page where this element is not used (to validate those non-nested sidebars didn't break) like https://pr-9047-documentation.appa.pantheon.site/guides/drush

Prior to merge, this PR needs

• Technical review from @mel-miller
• Styling adjustments from @mel-miller (The navbar element has styling that should perhaps be moved to the new OmniSidebarNav that wraps navbar)
• More details for planned follow up written out under Plan for full integration of nested sidebar #9084
• More guidance written into the readme of the new component or the top level readme on how to add adjust the menu items themselves.

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[rachelwhitton]

@stevector or @jazzsequence can you resolve merge conflicts with main here when convenient? Conflicts were introduced on this branch after I reverted Prettier on main yesterday, and I'm not familiar enough w/the changes in this PR to resolve

[stevector]

Yes, though I'll work on getting the Prettier change back into main first.

[pantheon-decoupled]

⚡ Deployed with Pantheon Decoupled

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👀 Preview: https://pr-9047-documentation.appa.pantheon.site
🛠️ Manage in Pantheon: https://dashboard.pantheon.io/site/2b30153f-e8b1-4427-b076-6109e704ba5d/overview