Rename the page for expiring Signon API tokens

This check is getting its own alert, so the page should reflect that.
The current page isn't linked from anywhere, so nothing else needs
updating.

source/manual/alerts/signon-app-healthcheck-not-ok.html.md → source/manual/alerts/expiring-api-tokens-in-signon.html.md

@@ -1,6 +1,6 @@
 ---
 owner_slack: "#govuk-2ndline"
-title: Signon app healthcheck not ok
+title: Expiring API tokens in Signon
 parent: "/manual.html"
 layout: manual_layout
 section: Icinga alerts
@@ -12,10 +12,6 @@ section: Icinga alerts
 [govuk-secrets]: https://github.com/alphagov/govuk-secrets
 [gds-api-adapters]: https://github.com/alphagov/gds-api-adapters/blob/master/lib/gds_api.rb
 
-See also: [how healthcheck alerts work on GOV.UK](app-healthcheck-not-ok.html)
-
-## Rotating API Tokens
-
 One or more tokens for API Users are about to expire. You should rotate
 expiring tokens to ensure the associated application keeps working.
 
@@ -64,4 +60,4 @@ for Publishing API expires in X days*. In this example, the API User is
 
 Finally, most applications should automatically restart when Puppet updates the
 token on each machine, but you may need to [do this manually][restart-app] so
-that it picks up the new token from the environment.
+that it picks up the new token from the environment.