Fix doc redirects (#39039)

* Fix doc redirects

* Fix other redirects + add explanations about "../"

(cherry picked from commit 40bddb3)

docs/apache-airflow/redirects.txt

@@ -19,8 +19,15 @@
 
 administration-and-deployment/security/index.rst security/index.rst
 administration-and-deployment/security/kerberos.rst security/kerberos.rst
-administration-and-deployment/security/access-control.rst ../../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
-administration-and-deployment/security/access-control/index.rst ../../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
+
+# Moving FAB auth manager related docs to providers
+## The ``../`` indicates that it will move to the root of the docs directory, unlike the rest of
+## the entries here which move to docs/apache-airflow.
+## It's okay to include ``/stable/``, because there's no relationship between a version of
+## Airflow and the version of any provider package.
+
+administration-and-deployment/security/access-control.rst ../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
+administration-and-deployment/security/access-control/index.rst ../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
 administration-and-deployment/security/api.rst security/api.rst
 administration-and-deployment/security/audit_logs.rst security/audit_logs.rst
 administration-and-deployment/security/flower.rst security/flower.rst
@@ -37,7 +44,7 @@ howto/use-alternative-secrets-backend.rst security/secrets/secrets-backend/index
 security.rst security/index.rst
 
 # Move the documentation from core to FAB provider
-security/access-control.rst ../../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
+security/access-control.rst ../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
 
 # Operators guides
 howto/operator/external.rst howto/operator/external_task_sensor.rst
@@ -47,7 +54,7 @@ howto/customize-dag-ui-page-instance-name.rst howto/customize-ui.rst#customizing
 howto/customize-state-colors-ui.rst howto/customize-ui.rst#customizing-state-colours
 
 # Web UI
-howto/add-new-role.rst ../../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
+howto/add-new-role.rst ../apache-airflow-providers-fab/stable/auth-manager/access-control.rst
 
 # Set up a database
 howto/initialize-database.rst howto/set-up-database.rst