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

[pytorchbot]

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/19605

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❌ 1 New Failure, 1 Unrelated Failure

As of commit 526314a with merge base 0cafcb2 ():

NEW FAILURE - The following job has failed:

• Lint / lintrunner-mypy (gh)
  >>> Lint for backends/arm/test/passes/test_symbolic_value_range.py:

FLAKY - The following job failed but was likely due to flakiness present on trunk:

• pull / test-qnn-testsuite-linux / test-backend-linux (qnn, models) / linux-job (gh) (detected as infra flaky with no log or failing log classifier)

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