docs: add VitePress documentation site with GitHub Pages deploy

[cB-Guru-Sharan-Kumar-Ram]

Documentation site for pagi-help, built with VitePress and hosted on GitHub Pages.

• Guide: getting started, installation, and a v1-to-v2 migration path
• Full v2 API pages: constructor, paginate(), cursor pagination, filters & operators, sorting, search, return shape, API reference
• MySQL/PostgreSQL dialect pages plus worked examples: filtering, searching, sorting, pagination, joins, unions, cursor
• Interactive demo and live playground run the published package (2.5.4) in the browser — explore generated SQL with zero setup
• Every documented SQL output is verified against the library by executing it
• Auto-deployed by GitHub Actions (deploy-site.yml); Dependabot keeps the bundled engine current