Exclude 404.html from lychee link checks

The 404 page contains root-relative links (/, /documentation/) that
cannot be resolved when lychee scans local files.  Lychee v0.21
reported these as warnings (pass), but v0.23 promotes them to errors
(fail), breaking the docs CI on PRs.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: sbryngelson

.lychee.toml

@@ -16,8 +16,11 @@ accept = ["200", "429", "502"]
 
 verbose = "error"
 
-# Exclude sitemap file from scanning (it contains pre-publish production URLs)
-exclude_path = ["sitemap\\.xml"]
+# Exclude files from scanning
+exclude_path = [
+    "sitemap\\.xml",  # Contains pre-publish production URLs
+    "404\\.html",     # Root-relative links (/, /documentation/) can't resolve in local checks
+]
 
 # Exclude URLs that fail due to external issues (broken SSL, pre-deploy only, etc.)
 exclude = [