docs: why we separate two backend-server, one in Spring Boot, one in NestJS

APITable has two backend servers.

One called "backend-server" with Java code and the Spring Boot framework.

The other is called "room-server" and includes NodeJS TypeScript code and the NestJS framework.

• Why do we separate servers' code into two programming languages?
• Have you ever seen a similar architect in another case?
• What are the philosophical differences between the NestJS framework and Spring Boot?

TIPS: related with product features.