fix link check (#5590)

PR Doc #5443 broke the link checker because it includes a link to this
private repo:

https://github.com/paritytech/devops/issues/3502

This PR adds the `prdoc` folder to the excluded paths of the link
checker to ensure that historical PR Docs do not break the pipeline.

I think this makes sense over whitelisting the github link because we
probably expect that in a long enough time period, links from old PR
docs will start to break, but I don't think we intend to update those
old PR docs, or start whitelisting lots of urls.

.config/lychee.toml

@@ -18,7 +18,10 @@ accept = [
 	"429",
 ]
 
-exclude_path = ["./target"]
+exclude_path = [
+	"./prdoc",
+	"./target",
+]
 
 exclude = [
 	# Place holders (no need to fix these):