docs: update docs on mirrors and registry map.

[Djabx]

Description

Update the documentation associated with this PR: https://github.com/GoogleContainerTools/kaniko/pull/3051

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you review them:

• [X] Includes unit tests
• [X] Adds integration tests if needed.

See the contribution guide for more details.

Reviewer Notes

• [ ] The code flow looks good.
• [ ] Unit tests and or integration tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

Examples of user facing changes:
- kaniko adds a new flag `--registry-repo` to override registry

[Djabx]

@prima101112 I let your type from your MR :)