Previously we generated the URL for the link in the help text based on
the user-submitted value in `datadog_site`, which caused a stored XSS
vulnerability.

This was accidentally fixed with the HTML-escaping we added in
https://gitlab.com/gitlab-org/gitlab/-/merge_requests/66391 while
improving the help texts for this integration.

But this still leaves some room for exploitation by changing the URL
of a seemingly-trusted link, so instead we just hard-code the default
link to the documentation on docs.datadoghq.com, and remove the custom
link which goes directly to the user's API settings.

Changelog: security