message: Add codeowners for techdocs-common package
diff: /docs/features/techdocs @backstage/techdocs-core
  /plugins/cost-insights @backstage/silver-lining
  /plugins/cloudbuild @trivago/ebarrios
  -/plugins/techdocs @backstage/techdocs-core
  /plugins/search @backstage/techdocs-core
  +/plugins/techdocs @backstage/techdocs-core
  /plugins/techdocs-backend @backstage/techdocs-core
  +/packages/techdocs-common @backstage/techdocs-core
  /.changeset/cost-insights-* @backstage/silver-lining
  