Certificate errors give a 404 link

[richvdh]

If there is a certificate error (particularly if you have a SRV record and a cert for the wrong domain), the UI links to https://github.com/matrix-org/synapse/blob/master/docs/MSC1711_certificates_FAQ.md#configuring-certificates-for-compatibility-with-synapse-100:

While that document is still technically correct, it's written from the perspective of people upgrading from Synapse 0.99 to Synapse 1.00, so isn't terribly helpful. 2023/01/16: it's now a 404.

Probably we should just link to https://matrix-org.github.io/synapse/latest/delegate.html. We might also want to add a small note about the gotcha under https://matrix-org.github.io/synapse/latest/delegate.html#srv-dns-record-delegation - "note in particular that you must present a certificate for the original server_name".

[babolivier]

Some additional info, the code for this warning is located here:

fed-tester-ui/app.js

Lines 74 to 80 in e51e89c

	if (!report.Checks.ValidCertificates) {
	tldr.push(<div className="warning" key={`cert-${tldr.length}`}>
	WARN: Could not find a valid certificate for {ip}.
	See <a href="https://github.com/matrix-org/synapse/blob/master/docs/MSC1711_certificates_FAQ.md#configuring-certificates-for-compatibility-with-synapse-100">this
	documentation</a> for instructions on how to fix this.
	</div>)
	}

[richvdh]

doubly important now that matrix-org/synapse#11907 has landed, making the link a 404