Added redirect and modified api reference section

docs/source/reference/api/auth.md

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+# Authenticators
+
+## Module: {mod}`jupyterhub.auth`
+
+```{eval-rst}
+.. automodule:: jupyterhub.auth
+```
+
+### {class}`Authenticator`
+
+```{eval-rst}
+.. autoconfigurable:: Authenticator
+   :members:
+```
+
+### {class}`LocalAuthenticator`
+
+```{eval-rst}
+.. autoconfigurable:: LocalAuthenticator
+   :members:
+```
+
+### {class}`PAMAuthenticator`
+
+```{eval-rst}
+.. autoconfigurable:: PAMAuthenticator
+```
+
+### {class}`DummyAuthenticator`
+
+```{eval-rst}
+.. autoconfigurable:: DummyAuthenticator
+```