Standardize header capitalization across documentation to follow
title case conventions (capitalize major words like nouns, verbs,
adjectives; keep minor words lowercase).
Changes include:
- Model provider docs: "Chat model" → "Chat Model", etc.
- Plan mode docs: "Understanding Plan mode" → "Understanding Plan Mode"
- CLI docs: "Quick start" → "Quick Start", "Next steps" → "Next Steps"
- Guide docs: various headers fixed for consistency
- Also fixed typo: "Availible models" → "Available Models"
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
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