correct misinformation about identity trust domain

In looking at https://github.com/linkerd/linkerd2/issues/9848#issuecomment-1681825662 with @olix0r today he noted that the statement that the identity trust domain must match the cluster domain is incorrect. We should fix the corresponding doc.

Start with the linked Linkerd issue comment to understand the incorrect identity trust domain statement, then search the website documentation for the corresponding wording. Update that documentation to remove the claim that the identity trust domain must match the cluster domain, and verify the corrected text in the generated site or documentation checks.

Written by the indexing model from the issue text.