ci: add website workflow to publish docs

[luoyuxia]

Purpose

Linked issue: close #148
Enable automated website publishing for paimon-rust so documentation updates can be built and deployed consistently on every push to main.

Brief change log

• Add .github/workflows/website.yml.
• Build docs with MkDocs from docs/mkdocs.yml.
• Use scheme B path handling:
  • run build in docs/
  • output to repo-root _site
  • copy .asf.yaml into _site/.asf.yaml
• Deploy built site to gh-pages branch via peaceiris/actions-gh-pages.
• Trigger strategy:
  • push to main: always run website build/deploy
  • pull_request: run only when docs/workflow files are touched

Tests

Already verify it in my own fork.
https://luoyuxia.github.io/paimon-rust/

API and Format

Documentation

[luoyuxia]

after discuss with @JingsongLi offline, we will publish in https://github.com/apache/paimon-website. I'll close this pr