fix(website): improve docs crawlability and sitemap targets

[PredictabilityAtScale]

Motivation

• Improve search engine and crawler visibility for the Docsify-hosted docs without removing the client-side Docsify experience.
• Ensure sitemap and robots directives point to crawler-friendly targets so launch-site crawlers index the docs correctly.

Description

• Added a crawler-visible static index and styling to website/docs/index.html that lists direct links to each documentation page while preserving the Docsify app for browsers.
• Inserted a canonical link pointing to https://promptopskit.com/docs/index.html in website/docs/index.html to provide a stable target for crawlers.
• Reworked website/sitemap.xml to target https://promptopskit.com/docs/index.html (instead of /docs/) and added explicit URLs for all main docs pages to increase index depth.
• Updated website/robots.txt to reference the production sitemap at https://promptopskit.com/sitemap.xml instead of the GitHub blob URL.

Testing

• Ran npm test which invokes vitest, and all tests passed (12 files, 170 tests).
• Verified the modified HTML and XML files were updated locally and contain the canonical, static links, and sitemap entries as intended.

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Codex Task