docs: add mongodbatlas app support

[danibarranqueroo]

Context

Provide MongoDB Atlas users with clear instructions for onboarding through Prowler App (not only the CLI), including screenshots and IP allowlist requirements.

Description

Expanded the MongoDB Atlas getting-started doc with the full Prowler App workflow—provider selection, credential entry, connection test, screenshots, and warnings about required IP allowlists—so cloud users know exactly how to connect Atlas via the UI alongside the existing CLI guidance.

Checklist

• Are there new checks included in this PR? Yes / No
  • If so, do we need to update permissions for the provider? Please review this carefully.
• [ ] Review if the code is being covered by tests.
• [ ] Review if code is being documented following this specification 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.

UI

• [ ] All issue/task requirements work as expected on the UI
• [ ] Screenshots/Video of the functionality flow (if applicable) - Mobile (X < 640px)
• [ ] Screenshots/Video of the functionality flow (if applicable) - Table (640px > X < 1024px)
• [ ] Screenshots/Video of the functionality flow (if applicable) - Desktop (X > 1024px)
• [ ] Ensure new entries are added to CHANGELOG.md, if applicable.

API

• [ ] Verify if API specs need to be regenerated.
• [ ] Check if version updates are required (e.g., specs, Poetry, etc.).
• [ ] Ensure new entries are added to CHANGELOG.md, if applicable.

License

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

[github-actions[bot]]

✅ Conflict Markers Resolved

All conflict markers have been successfully resolved in this pull request.

[github-actions[bot]]

✅ All necessary CHANGELOG.md files have been updated.