Document the workflow for reviewing Translation MRs

We need to establish and document a clear review workflow for Translation MRs. This workflow will define and set clear expectations for all stakeholders involved in the process, for how to collaborate to review, approve, and merge translated content.

Goal

• Define clear roles and responsibilities for all stakeholders involved in the process:
  • Localization team (managing translation flow and fork repositories)
  • Japanese reviewers (verifying technical integration and structural integrity)
  • Technical maintainers (approving and merging to upstream repositories)
• Create comprehensive documentation that:
  • outlines the process for reviewing Translation MRs in forked repositories
  • defines the process for syncing approved translations to upstream repositories
  • specifies when and how maintainers should be involved
  • clarifies at what earlier stages the JA linguistic quality is addressed 
• Store and iterate on the documentation in .md format in https://gitlab.com/gitlab-com/localization/docs-site-localization (to be later transferred into Handbook)
• Include guidelines for how to review Translation MRs (focusing on technical aspects, not linguistic review), how to resolve merge conflicts (if any), how not to make target updates but defer to Define target updates use cases in tech docs (#55)
• Document the separation of responsibilities where, for example:
  • Localization team members maintain forks, but don't have rights to merge to upstream
  • Maintainers merge to upstream, but don't evaluate Japanese content