Enforce docs heading hierarchy

[Alek99]

What changed

• Promoted the Modebars page's top-level sections from H3 to H2.
• Promoted the benchmark demo heading to H2 so visible demo content fits the page outline.
• Added an all-public-route heading-order validator plus a negative first-H3 regression test.

Why

The two affected routes skipped directly from H1 to H3, weakening the semantic outline used by assistive technology and generated navigation.

Impact

Public documentation headings now advance by at most one level, and future route-level heading jumps fail the docs suite.

Validation

• docs/app/.venv/bin/pytest -q docs/app/tests/test_heading_outline.py — 2 passed
• docs/app/.venv/bin/python docs/app/scripts/check_heading_outline.py
• Ruff lint and format checks on changed Python files

Fixes #397

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Review details

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro Plus

Run ID: 2cd99f37-dad8-4efe-813f-ceae28ebcfc1

📥 Commits

Reviewing files that changed from the base of the PR and between d505ef5 and 8e2bf3e.

📒 Files selected for processing (4)

• docs/app/scripts/check_heading_outline.py
• docs/app/tests/test_heading_outline.py
• docs/app/xy_docs/demos/benchmark_charts.py
• docs/components/modebars-and-interaction-controls.md

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[cubic-dev-ai]

All reported issues were addressed across 4 files

<sub>Reply with feedback, questions, or to request a fix.

Fix all with cubic | Re-trigger cubic</sub>