docs: add MCP server documentation

New feature page covering architecture, available tools, local/HTTP
usage, Docker deployment, Coolify setup, dashboard management, and
example queries. Updated environment variable reference and
configuration guide with MCP_PORT. Added sidebar nav entry.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/.vitepress/config.mts

@@ -33,6 +33,7 @@ export default defineConfig({
           { text: 'DAST Scanning', link: '/features/dast' },
           { text: 'AI Chat (RAG)', link: '/features/ai-chat' },
           { text: 'Issue Tracker Integration', link: '/features/issues' },
+          { text: 'MCP Server', link: '/features/mcp-server' },
         ],
       },
       {