Revert "Upgrade dartdoc to 0.30.1 (#50980)" (#51060)

This reverts commit a51de38a because it breaks the flutter docs.

dev/bots/docs.sh

@@ -111,7 +111,7 @@ if [[ -d "$FLUTTER_PUB_CACHE" ]]; then
 fi
 
 # Install and activate dartdoc.
-"$PUB" global activate dartdoc 0.30.1
+"$PUB" global activate dartdoc 0.29.3
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.