docs: update app icons on core template

[unicoderbot]

Warning

The coding agent reported this implementation is incomplete.
Documented icon updates for the core template; wear-app docs were skipped because the wear template is deprecated upstream and not exposed by the CLI.

This PR has been opened as a draft. Review the diff against the original issue before marking it ready for review.

Summary

Adds comprehensive documentation on how to update application icons in the Very Good Core template, covering both the recommended automated approach using flutter_launcher_icons and manual platform-specific updates.

Changes

• Added section on using flutter_launcher_icons package with step-by-step instructions for setup and generation across all flavors
• Documented manual icon update procedures for Android, iOS, macOS, Web, and Windows platforms
• Included flavor-specific guidance since Very Good Core ships with separate icons for development, staging, and production
• Added configuration examples and verification steps
• Added links to relevant documentation and guidelines

References

Closes #768

[marcossevilla]

@vgvbot investigate and fix CI

[unicoderbot]

On it, I'll address the review comments and push the changes.

• Preparing environment
  • Cloning repository
  • Setting up dev environment
• Reviewing feedback
• Working on changes
• Pushing updates

I've pushed changes to address the feedback.