Publish the documentation as a VitePress site on GitHub Pages

[zorenkonte]

Summary

Moves the long-form documentation out of the README into a VitePress site under docs/, deployed to the GitHub Pages root. The hosted web client moves from the Pages root to /tibeepost/app/. The copy the TV serves at http://TV_IP:8090/ is unchanged.

Changes

• docs/: VitePress 1.6 site with a home page, Guide (getting started, TV settings, web client, troubleshooting), API (HTTP reference with the full field table, notification behavior) and Development pages. Green brand theme matching the TV app, local search, edit links, dead-link checking.
• pages.yml: builds the client with VITE_BASE=/tibeepost/app/, builds the docs, places the client under /app/ of the docs output and deploys the combined folder. Also triggers on docs/**.
• ci.yml: new docs job that builds the site so a broken cross-page link fails the PR.
• README.md: trimmed to an overview with install, the permission command, the curl quickstart and a link per docs page. The obsolete Branches section is gone.
• client/README.md: hosted URL and VITE_BASE example point at /tibeepost/app/.

Testing

• cd docs && npm run build passes with zero dead links.
• Built the client with VITE_BASE=/tibeepost/app/, copied it into the docs output, served the combined folder with vitepress preview and loaded /tibeepost/, /tibeepost/api/, /tibeepost/guide/web-client and /tibeepost/app/ in Chromium: all four render, the client shows its Compose, Devices, History and curl tabs, and no request other than the browser's implicit favicon probe returns 404.
• No TV or client source changed; tv/app/build.gradle.kts still builds the bundled copy with VITE_BASE=/.

After merging, the Pages deploy should be confirmed at https://zorenkonte.github.io/tibeepost/ and https://zorenkonte.github.io/tibeepost/app/.

Checklist

• cd tv && ./gradlew testDebugUnitTest assembleDebug passes (no TV files changed; CI runs it)
• cd client && npm run lint && npm run build passes (no client source changed; build verified under the new base)
• No comments added to source files
• README updated when behavior, the API or setup changed
• Each commit is one logical change and builds on its own