Add AIGW SSL Setup Instructions

What does this MR do and why?

This MR adds steps for installing SSL on an AI Gateway (AIGW) instance using Docker, Nginx as a reverse proxy, and Let's Encrypt for SSL certificates.

Discussion here: gitlab-org/modelops/applied-ml/code-suggestions/ai-assist#854 (comment 2436817139)

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

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