Consolidate and improve FIELD_STANDARDIZATION.md

Problem

The .labkit_logging_todo.yml baseline file (added in gitlab-org/gitlab!222748 (merged)) embeds an inline "HOW TO FIX" guide that duplicates content from labkit-ruby's doc/FIELD_STANDARDIZATION.md, as noted by @splattael in this review comment. The doc itself was also missing lifecycle documentation and did not follow the GitLab documentation style guide.

Goal

  1. Consolidate the inline guide into doc/FIELD_STANDARDIZATION.md as the single source of truth
  2. Add missing sections for the full field deprecation lifecycle
  3. Align with GitLab documentation style guide
  4. Improve troubleshooting guidance for CI failures

Related

Edited by Doug Barrett