message: docs: add backend-to-backend auth tutorial to microsite
diff: "tutorials/migrating-away-from-core",
  "tutorials/configuring-plugin-databases",
  "tutorials/switching-sqlite-postgres",
  - "tutorials/using-backstage-proxy-within-plugin"
  + "tutorials/using-backstage-proxy-within-plugin",
  + "tutorials/backend-to-backend-auth"
  ],
  "Architecture Decision Records (ADRs)": [
  "architecture-decisions/adrs-overview",
  