feat(website): enhanced control over navbar and sidebar titles (#13477)

[jtbayly]

Description

• Allow navbar.title and sidebar.title to be boolean or string.
• Fix bug where disabling navbar title erroneously hid sidebar title. Title hidden in sidebar when disabled in navbar #13477
• Support custom titles for navbar and sidebar.
• Add smoke tests for title control options.
• Update schema documentation for titling options.

Checklist

I have (if applicable):

• filed a contributor agreement.
• referenced the GitHub issue this PR closes
• updated the appropriate changelog in the PR (Not yet. Waiting for positive feedback.)
• ensured the present test suite passes
• added new tests
• created a separate documentation PR in Quarto's website repo and linked it to this PR (I'll work on this if I get positive feedback on this.)

[posit-snyk-bot]

✅ Snyk checks have passed. No issues have been found so far.

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✅	Licenses	0	0	0	0	0 issues

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[jtbayly]

I did a lot of testing to make sure that every single combination of options works as explained in my proposal on the bug report. I compared screenshots of all the combinations, to make sure that nothing changed except for fixing this bug, and the small feature additions.