Suppress checking links in the external folder

Author: vmoroz

.unbroken_exclusions

@@ -22,5 +22,6 @@ URL not found https://aka.ms/watson while parsing docs/sdl-gs-crash-monitoring.m
 !packages/@rnw-scripts/doxysaurus/node_modules
 !packages/@rnw-scripts/format-files/node_modules
 !packages/@rnw-scripts/promote-release/node_modules
+!vnext/external
 !.github/ISSUE_TEMPLATE
 !.github/security.md