Semantic versioning for Self-managed AI Gateway

What does this merge request do and why?

This MR introduces semantic versioning for Self-managed AI Gateway. See docs/release.md for more information.

Tested in https://gitlab.com/shinya.maeda/ai-assist/-/pipelines and confirmed it's working.

• https://gitlab.com/shinya.maeda/ai-assist/container_registry
• https://gitlab.com/shinya.maeda/ai-assist/-/releases

Related to Semantic versioning for AI Gateway (#431 - closed)

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

Merge request checklist

• Tests added for new functionality. If not, please raise an issue to follow up.
• Documentation added/updated, if needed.