docs(image): add Image provider CLI documentation

[andoniaf]

Stacked PR — targets feat/PROWLER-939-stage-1-image-provider-mvp-for-cli (#9984), not master.

Context

Adds user-facing documentation for the new Image (container image) provider introduced in #9984.

Description

• Add getting-started-image.mdx with overview, prerequisites, installation, usage, scan configuration, output options, and private registry authentication
• Structure follows the IaC provider pattern: ## Prowler CLI section with <VersionBadge> and a <Note> indicating CLI-only availability
• Register the Image provider page in docs.json navigation (between Cloudflare and LLM)

Steps to review

1. Review docs/user-guide/providers/image/getting-started-image.mdx for accuracy and completeness
2. Review docs/docs.json for correct navigation placement
3. Verify the page renders correctly in Mintlify (preview deploy)

Checklist

Community Checklist

• This feature/issue is listed in here or roadmap.prowler.com
• Is it assigned to me, if not, request it via the issue/feature in here or Prowler Community Slack

• Are there new checks included in this PR? No
• Review if the code is being covered by tests.
• Review if code is being documented following https://github.com/google/styleguide/blob/gh-pages/pyguide.md#38-comments-and-docstrings
• Review if backport is needed.
• Review if is needed to change the Readme.md
• Ensure new entries are added to CHANGELOG.md, if applicable.

SDK/CLI

• Are there new checks included in this PR? No

License

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.