[Documentation:System] Added documentation for configuring custom websocket port in NGINX

[lalithaar]

Summary:

• Added a new section under NGINX configuration to explain how to set up a custom websocket port.
• Included steps to modify the submitty.json file and update the proxy_pass directive in the NGINX configuration.
• Clarified default behavior if the websocket_port field is not set.

Why this change is needed:

• Previously, the documentation lacked details for configuring custom websocket ports after #6814 was merged.
• An issue was created on here. Fixes #6813
• Helps users avoid conflicts when running multiple instances or using different environments.

Changes made:

• Added a new "Configure Websocket Port" section to the NGINX setup documentation.
• Provided clear step-by-step instructions and examples.

Testing:

• Documentation changes only, no code changes.
• Verified formatting and clarity.