[mdns] Renamed mDNS abstract layer to DNS-SD (#10381)

Abstract layer providing DNS Service Discovery functionalities
is currently called mDNS, but mDNS is actually a DNS-SD specific
implementation. Abstract layer should be named DNS-SD, while
mDNS and SRP should be modules implementing DNS-SD API.

Overall 1:1 changes from mDNS naming to dnssd, including:
* src/lib/mdns -> src/lib/dnssd
* lib/mdns/minimal -> lib/dnssd/minimal_mdns
* app/server/Mdns -> app/server/Dnssd
* MdnsImpl -> DnssdImpl
* Mdns namespace -> Dnssd namespace
diff --git a/docs/Doxyfile b/docs/Doxyfile
index 03b810c..2793994 100644
--- a/docs/Doxyfile
+++ b/docs/Doxyfile
@@ -836,7 +836,7 @@
                          src/crypto \
                          src/include \
                          src/inet \
-                         src/lib/mdns \
+                         src/lib/dnssd \
                          src/lib/core \
                          src/lib/support \
                          src/lib/shell \