docs: add community registry and widget deployment documentation

[neuromechanist]

Summary

• Add comprehensive community registry documentation (YAML-driven assistant configuration)
• Add widget deployment guide with all configuration options
• Update navigation and OSA index page

New Documentation

Community Registry (docs/osa/registry/)

• Overview - How the registry works, directory structure, key principles
• Quick Start - Step-by-step tutorial for adding a new community assistant
• Schema Reference - Full YAML config reference with all fields, types, and validation rules
• Extensions - Python plugin and MCP server extension guide

Deployment (docs/osa/deployment/)

• Widget - Embeddable chat widget configuration, features, and troubleshooting

Related

Closes OpenScience-Collective/osa#49 (Phase 6: Documentation for YAML registry system)

Test plan

• Verify all new pages render correctly in mkdocs
• Check all internal links resolve
• Confirm nav structure matches issue requirements

[neuromechanist]

PR Review Findings - Addressed

Review ran with comment-analyzer and code-reviewer agents. All critical and important issues fixed in commit 11f84ca.

Critical (Fixed)

• Plugin discovery docs incorrect: Claimed @tool scanning; actually uses __all__ exports
• Widget markdown claims: Removed false claims of syntax highlighting and strikethrough
• Missing source_repo field: Added to DocSource schema reference table
• Widget API response format: Fixed from session_id to tool_calls (matching AskResponse model)

Important (Fixed)

• cf_turnstile_response: Added note that it's consumed by Worker proxy, not backend
• Missing widget config options: Added repoUrl, repoName, pageContextStorageKey, pageContextLabel
• communityId validation: Updated troubleshooting to match actual regex (accepts uppercase and underscores)
• Extensions loading order: Corrected to match actual init sequence in CommunityAssistant