Fix header formatting: shorten nav title and prevent nav items from w…

[Nazim-fad]

Summary

Fix header formatting #19347

The navigation bar header had inconsistent spacing due to nav items wrapping to multiple lines on most screens.

Changes:

• Shortened "Find Your Path in ExecuTorch" → "Find Your Path" in user-pathways.md
• Added docs/source/_static/navbar.css with white-space: nowrap to prevent nav items from wrapping in the header
• Registered navbar.css in conf.py via html_css_files

Test plan

Built docs locally and previewed:

Ran lintrunner on changed files

cc @GregoryComer @AlannaBurke @mergennachin

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/19441

• 📄 Preview Python docs built from this PR

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❌ 1 New Failure, 1 Cancelled Job, 8 Pending

As of commit a9e7f64 with merge base a49171d ():

NEW FAILURE - The following job has failed:

• Docathon Labels Sync / check-labels (gh)
  Process completed with exit code 1.

CANCELLED JOB - The following job was cancelled. Please retry:

• periodic / gather-models (gh)

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[GregoryComer]

Looks good. Thanks for the contribution!