fix: reduce lychee retries to avoid compounding GitHub 429s (#1940)

## Summary

- Reduce `max_retries` from 6 to 2 and increase `retry_wait_time` from
5s to 10s
- 6 retries with 5s wait compounds 429 rate limiting from GitHub — many
rapid retries keep triggering rate limits
- Fewer retries with longer wait between them is more effective at
recovering from 429s

## Test plan

- [ ] Link check CI workflow passes without hitting sustained 429 errors

Author: zeitlinger

.github/config/lychee.toml

@@ -2,8 +2,8 @@
 # See https://lychee.cli.rs/config/
 
 timeout = 30
-retry_wait_time = 5
-max_retries = 6
+retry_wait_time = 10
+max_retries = 2
 max_concurrency = 4
 
 # Check link anchors