Rewrite homepage content and structure in website/index.html

[PredictabilityAtScale]

Motivation

• Improve clarity and positioning by updating the homepage messaging to emphasize prompts-as-versioned-assets and runtime integration.
• Reorganize content so core concepts (prompt file, runtime, testing, composition, providers) are easier to find and review.
• Clean up examples and code snippets to show a compact prompt file and provider-ready rendering with bring-your-own transport.

Description

• Updated website/index.html copy including meta description, Open Graph and Twitter descriptions, hero headline, lead paragraphs, and multiple section headings to better reflect PromptOpsKit's value proposition.
• Reworked navigation links and reordered/renamed sections (added #prompt-file, #runtime, #composition, #providers, moved #testing, and consolidated feature/FAQ content) and removed duplicated or outdated blocks.
• Replaced and simplified example prompt front matter and runtime/rendering snippets, clarified testing sidecar examples, and adjusted CTAs and demo text; also applied minor HTML formatting/whitespace tweaks (anchor spacing, canonical link formatting).

Testing

• No automated tests were run for this content-only change.

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Codex Task