ci: Exclude Slack invite URLs from link checker

Slack's join.* endpoints reject automated GETs
and return 403, so lychee was failing the link availability
check on any PR that touched README.md

Add the join.slack.com/t/ prefix to the .lychee.toml
exclude list

Signed-off-by: Saravanan D <saravanand@crusoe.ai>

.lychee.toml

@@ -21,6 +21,8 @@ exclude = [
   '^https://developer\.arm\.com',
   # Ignore internal/unsupported protocols seen in logs
   '^tcp://192\.168\.1\.10',
+  # Slack invite endpoints reject automated GETs and return 403.
+  '^https://join\.slack\.com/t/',
 ]
 
 # Exclude loopback addresses