Document how to use other identity trust root

```bash
$ step certificate create --profile root-ca "Example Root CA" root-ca.crt root-ca.key --no-password --insecure
$ step certificate create identity.linkerd.cluster.local identity.crt identity.key --profile intermediate-ca --ca ./root-ca.crt --ca-key ./root-ca.key --no-password --insecure
$ linkerd install --identity-trust-anchors-file root-ca.crt --identity-issuer-key-file identity.key --identity-issuer-certificate-file identity.crt
```

Use the three provided Bash commands as the starting example for the documentation. Locate the website's identity or installation guidance, add an explanation of using a separate trust root and identity issuer, and verify that the documented steps clearly cover certificate creation and Linkerd installation.

Written by the indexing model from the issue text.