Add information about using JSON for HTTPS queries

[ameshkov]

AdGuard DNS supports JSON for DoH (like Google does), but it's not documented anywhere save for the code: https://github.com/AdguardTeam/AdGuardDNS/blob/41f7e6cb220095a9d8955e5ed0167dce60616d65/internal/dnsserver/serverhttpsjson.go#L13

We should expand and improve the protocols documentation: https://adguard-dns.io/kb/public-dns/overview/#adguard-dns-protocols