Update Desktop Client documentation around client certificate options

[kenjenkins]

Page: https://www.pomerium.com/docs/capabilities/tcp/client#add-a-connection

What's incorrect or missing

Desktop Client v0.23.0 adds a new "Search OS certificate store" option for client certificates, on Windows and macOS.

What's the resolution?

There's a brief mention of client certificates on this page:

Client Certificate & Certificate Key File or Text: For routes that require client certificates for mTLS, you can provide the certificate and key file to the Pomerium Desktop client.

Let's also mention the new option here.

While we're here, we might also want to update the Desktop Client screenshot as well, as it's a few versions out of date.