fix(docs): enhance gcp service account authentication and add missing permissions

[HugoPBrito]

Context

Expands the GCP provider docs so the Service Account Key File Path mirrors the workflow security teams should actually follow when provisioning least-privilege identities for prowler, also adding missing permissions.

Description

• Rewrote docs/user-guide/providers/gcp/authentication.mdx to provide full motivation for service-account usage, add role-preparation guidance, and document each wizard step (role creation, account creation, role binding, key generation, key handling).
• Added the supporting screenshots under docs/user-guide/providers/gcp/img/ (roles-section.png, prowler-role.png, service-account-page.png, create-service-account.png, service-account-permissions.png, create-new-key.png, json-key.png) and linked them inline so readers can visually confirm they are in the right console view.
• Highlighted the specific storage.buckets.getIamPolicy permission inside the ProwlerRole example to match the captured UI and make the least‑privilege requirement explicit.
• No code or dependency changes; docs-only update.

Steps to review

1. npm run docs:dev (or your preferred docs preview) and open /user-guide/providers/gcp/authentication.
2. Scroll to Service Account Key File and confirm the new context paragraphs, numbered steps, and command snippets read cleanly and follow the project voice guidelines.
3. Verify each embedded image renders (especially the seven new files under docs/user-guide/providers/gcp/img/) and that alt text matches the screenshot context.
4. If desired, lint links/static assets with make lint-docs to ensure no broken references were introduced.

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]

✅ Conflict Markers Resolved

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

[github-actions]

✅ All necessary CHANGELOG.md files have been updated.