Improved tutorial documentation for parts 1 and 2: MongoDB setup with alternative port configuration, multi-LLM integration and path handling adjustments

[Jasmine-0425]

Description

This pull request addresses three main issues: 1) Providing an alternative port (27018) setup option based on common port conflicts with the default port (27017) faced by users. The changes were made in 1_setting-up-jac-cloud.md . 2) Integrating multiple LLMs into separate backend files (server.jac and gpt_server.jac) to avoid conflicts. 3) Resolving path conflicts with directory names containing special characters like \U. These changes were made in 2_building-a-rag-chatbot.md files to improve the clarity and smooth setup for new users.

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