[Feature] Add VitePress documentation site with GitHub Pages deployment

[DrSmile444]

What

Adds a full 42-page standalone documentation site under site/ covering every public export of @grammyjs/testing, deployed automatically to https://drsmile444.github.io/grammy-testing/ on every push to main.

Why

The library reached v0.22 with a rich two-layer API, 50+ dispatch verbs, and plugin integration patterns — but had no browsable documentation. Users had to read source code to understand the API, slowing adoption.

Changes

Documentation site (site/)

• site/.vitepress/config.ts — full VitePress config: dynamic GitHub Pages base URL, nav/sidebar with 6 groups, local search, social links, version badge from package.json
• site/.vitepress/theme/ — custom theme extending VitePress default; blue brand palette (#0057b7 light / #4d9eff dark), both WCAG AA compliant
• site/public/logo.svg — Y.svg (grammY Y mark) as site logo and favicon
• site/index.md — hero landing page with feature cards and install snippet
• Guide section (6 pages): Introduction, Getting Started, How It Works, With Vitest, With Jest, With Deno
• High-Level API section (9 pages): Overview, Chats, User, Groups, Channels, PrivateChat, BusinessAccount, Reply, Logs
• Low-Level API section (5 pages): Overview, Outgoing Requests, Session Mocking, Update Builders, Response Mocking
• Recipes section (7 pages): Sessions & State, Keyboards & Buttons, Error Simulation, Multi-Chat Scenarios, Conversations Plugin, Menu Plugin, Fire & Forget
• API Reference section (14 pages): full typed signatures for every exported symbol
• site/reference/changelog.md — symlink to docs/CHANGELOG.md

CI/CD

• .github/workflows/docs.yml — builds site/ and deploys to GitHub Pages on push to main or manual dispatch

Package

• package.json / jsr.json — version bumped to 0.22.0; added docs:dev, docs:build, docs:preview scripts; added vitepress devDependency
• .gitignore — added site/.vitepress/cache and site/.vitepress/dist
• eslint.config.mjs — added site/** to ignore patterns (VitePress config files have a separate TS context)

Housekeeping

• Import ordering fixed across all 22 example files (auto-fixed by lint)
• OpenSpec change archived; vitepress-site and documentation-content specs synced to openspec/specs/
• docs/CHANGELOG.md updated with 0.22.0 entry