docs: comprehensive rewrite of authentication documentation (#335)

- Add hero section with full-stack auth snippets (Frontend/API/Agent)
- Document default plugins (organization, jwt, bearer, apiKey)
- Expand middleware docs with hasOrgRole, hasPermission options
- Add full auth context (c.var.auth/ctx.auth) method reference
- Document agent-to-agent auth propagation
- Expand environment variables with defaults and auto-resolution
- Add CLI commands reference (auth init, auth generate)
- Link to BetterAuth docs for organization, API key, JWT plugins
- Link to SDK testing app for complete examples

Author: rblalock