feat: add custom error message on signups disabled page (#11959)

cli/testdata/coder_server_--help.golden

@@ -426,6 +426,10 @@ OIDC OPTIONS:
       --oidc-icon-url url, $CODER_OIDC_ICON_URL
           URL pointing to the icon to use on the OpenID Connect login button.
 
+      --oidc-signups-disabled-text string, $CODER_OIDC_SIGNUPS_DISABLED_TEXT
+          The custom text to show on the error page informing about disabled
+          OIDC signups. Markdown format is supported.
+
 PROVISIONING OPTIONS: 
 Tune the behavior of the provisioner, which is responsible for creating,
 updating, and deleting workspace resources.

cli/testdata/server-config.yaml.golden

@@ -348,6 +348,10 @@ oidc:
   # URL pointing to the icon to use on the OpenID Connect login button.
   # (default: <unset>, type: url)
   iconURL:
+  # The custom text to show on the error page informing about disabled OIDC signups.
+  # Markdown format is supported.
+  # (default: <unset>, type: string)
+  signupsDisabledText: ""
 # Telemetry is critical to our ability to improve Coder. We strip all personal
 # information before sending data to our servers. Please only disable telemetry
 # when required by your organization's security policy.