Add info about retagging images for publishing to other registries

[thokra-nav]

No description provided.

[Copilot]

Pull Request Overview

This PR adds documentation on retagging images for usage outside Nais and updates existing documentation to clarify registry usage.

• Adds a new guide ("use-image-outside-nais.md") detailing how to push images to the GitHub Container Registry.
• Updates "build-and-deploy.md" to adjust messaging regarding the Nais registry and its intended usage.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
docs/build/how-to/use-image-outside-nais.md	New document describing how to retag and push images to an external registry, including a YAML snippet for implementation.
docs/build/how-to/build-and-deploy.md	Updated informational box and text clarifying that the Nais registry is intended for internal use only.

Pull Request Overview

This PR adds documentation to guide users on how to retag and push container images to a registry outside the Nais platform. It introduces a new guide for using images outside of Nais and updates the build-and-deploy guide to clarify registry usage.

• Added a new how-to guide for retagging images to push to the GitHub Container Registry.
• Updated references in the build-and-deploy guide from "Github Actions" to "GitHub Actions" and clarified registry limitations.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
docs/build/how-to/use-image-outside-nais.md	New documentation on retagging and pushing images to GitHub Container Registry.
docs/build/how-to/build-and-deploy.md	Updated text and links to reflect correct registry usage and naming.