docs: add complete guide, env files, fix docker-compose

- Add GUIDE.md: complete user documentation covering installation,
  Docker deployment, AI providers, MCP server, N8N integration,
  email config, admin panel, env var reference, troubleshooting
- Add mcp-server/.env.example with all MCP-specific variables
- Update .env.docker.example with all 42 environment variables
- Fix docker-compose.yml: parameterize PostgreSQL credentials,
  add missing env vars (CUSTOM_OPENAI, AI_PROVIDER_CHAT,
  ALLOW_REGISTRATION, RESEND_API_KEY)
- Track memento-note/.env.example

memento-note/auth.config.ts

@@ -5,7 +5,7 @@ export const authConfig = {
     signIn: '/login',
     newUser: '/register',
   },
-  secret: "csQFtfYvQ8YtatEYSUFyslXdk2vJhZFt9D5gav/RJQg=",
+  secret: process.env.NEXTAUTH_SECRET,
   trustHost: true,
   session: {
     strategy: 'jwt',