Do more retries in linkcheck because NIST websites are down a lot (#3683)

diff --git a/docs/conf.py b/docs/conf.py
index 4539d48..80ac59f 100644
--- a/docs/conf.py
+++ b/docs/conf.py
@@ -176,7 +176,7 @@
 
 # Retry requests in the linkcheck builder so that we're resillient against
 # transient network errors.
-linkcheck_retries = 2
+linkcheck_retries = 5
 
 linkcheck_ignore = [
     # Certificate is issued by a Japanese CA that isn't publicly trusted