Document how to setup an SSL certificate for self hosted AIGW

We are currently missing documentation on how to enable SSL for self-hosted AIGW.

SSL certificates an important requirement for security, and we should document how we can add an SSL certificate for the AIGW endpoint in https://docs.gitlab.com/install/install_ai_gateway/.

Similar documentation for GitLab: https://docs.gitlab.com/omnibus/settings/ssl/

Ask

• As first cut, we could document how to enable SSL for AIGW deployed on Docker.
• To keep things simple, we can document the steps when we use LetsEncrypt as the SSL provider.