docs: Add redirects for the 73 page URLs that moved between 3.4.0 and 4.0

[FXschwartz]

The 4.0 restructure moved 77 of the 131 page URLs in 3.4.0; 73 have no entry in redirects.js. They still resolve today only because 3.4.0 is the default version at /.

Once 4.0.0 is cut as the default they 404. Two are printed by the framework itself: migration SQL links /upgrading/upgrade-to-pgvector (tools/serverpod_cli/lib/src/database/dialects/postgres.dart:760), and the template's greeting_endpoint.dart:24 plus the auth IDP base-endpoint doc comments link /concepts/working-with-endpoints. The real-time tutorial also links /serverpod-mini.

The redirects must land before the version cut. Computed by diffing slugs in versioned_docs/version-3.4.0 against docs/ (including explicit slug: frontmatter) and redirects.js.

• Add a redirect for each path below to its 4.0 location.
• Pages with no 4.0 equivalent (serverpod-mini, overview, concepts/experimental, the four deployments/deploying-to-* pages) need a target decision: nearest page, or the /3.4.0/… versioned copy.

73 paths

concepts/authentication/providers/{anonymous,apple,facebook,github,google,microsoft}/configuration
concepts/authentication/providers/{anonymous,apple,facebook,github,google,microsoft,passkey}/customizing-the-ui
concepts/backward-compatibility
concepts/caching
concepts/configuration
concepts/database/{crud,filter,indexing,migrations,models,pagination,raw-access,relation-queries,row-locking,runtime-parameters,sort,transactions}
concepts/database/relations/{many-to-many,modules,one-to-many,one-to-one,referential-actions,self-relations}
concepts/exceptions
concepts/experimental
concepts/file-uploads
concepts/health-checks
concepts/logging
concepts/models
concepts/modules
concepts/scheduling/recurring-task
concepts/security-configuration
concepts/serialization
concepts/server-events
concepts/sessions
concepts/shared-packages
concepts/streams
concepts/testing/{best-practises,the-basics}
concepts/webserver/{flutter-web,middleware,overview,request-data,routing,server-side-html,single-page-apps,static-files}
concepts/working-with-endpoints
deployments/{community-supported-deployments,deploying-to-aws,deploying-to-gce-terraform,deploying-to-gcr-console,deployment-strategy,general}
overview
serverpod-mini
tutorials/academy
tutorials/tutorials/{ai-and-rag,real-time-communication}
upgrading/{upgrade-from-mini,upgrade-to-pgvector,upgrade-to-three}