docs: add internal link to client connections docs

In the client connection [docs](https://github.com/canonical/valkey-operator/blob/9/edge/docs/how-to/clients.md#enable-authentication-through-client-certificate), we refer to `mutual TLS as a prerequisite for authentication through certificate`. This part should be linked internally to the `mutual TLS` section just prior to that, using an internal link anchor as in [this example](https://github.com/canonical/kafka-operator/blob/c2fe594f29f0f83f89feeedc8e661b20190b024a/docs/how-to/monitoring.md?plain=1#L15).

Open docs/how-to/clients.md and locate the mutual TLS section and the later reference to authentication through a client certificate. Compare the internal-anchor syntax with the linked kafka-operator example; done means that reference links to the preceding mutual TLS section and the documentation remains valid.

Written by the indexing model from the issue text.