Draft: Docs: Update Service Account IAM Roles and Add Step to Enable OS Login

What does this MR do?

This MR clarifies some missing steps in the GitLab Environment Toolkit documentation when working with GCP's OS Login. The Compute OS Admin Login IAM role is needed in order for the service account to login to an instance using OS Login. I have also added an extra step to better clarify how to enable OS Login for the project.

These steps were also already listed in external links, such as the blog post by Alex Dzyoba, but this helps make it more visible by adding them to the GET docs directly.

Related issues

Author's checklist

When ready for review, the Author applies the workflowready for review label and mention @gl-quality/get-maintainers:

• Merge request:
  • Corresponding Issue raised and reviewed by the GET maintainers team.
  • Merge Request Title and Description are up-to-date, accurate, and descriptive
  • MR targeting the appropriate branch
  • MR has a green pipeline
  • MR has no new security alerts in the widget from the Secret Detection and IaC Scan (SAST) jobs.
• Code:
  • Check the area changed works as expected. Consider testing it in different environment sizes (1k,3k,10k,etc.).
  • Documentation created/updated in the same MR.
  • If this MR adds an optional configuration - check that all permutations continue to work.
  • For Terraform changes: set up a previous version environment, then run a terraform plan with your new changes and ensure nothing will be destroyed. If anything will be destroyed and this can't be avoided please add a comment to the current MR.
• Create any follow-up issue(s) to support the new feature across other supported cloud providers or advanced configurations. Create 1 issue for each provider/configuration. Contact the Quality Enablement team if unsure.