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/deployment/environment.md

@@ -75,6 +75,15 @@ REDIRECT_URI=http://localhost:8080/auth/callback
 APP_URL=http://localhost:8080
 ```
 
+## MCP Server
+
+```bash
+MONGODB_URI=mongodb://root:example@localhost:27017/compliance_scanner?authSource=admin
+MONGODB_DATABASE=compliance_scanner
+# Set to enable HTTP transport (omit for stdio)
+MCP_PORT=8090
+```
+
 ## Observability
 
 ```bash