circleci: Skip checking links in documentation for now.

This is fairly often -- though not always! -- failing, with a nasty
failure mode where it takes like 6 minutes to time out.  See
discussion on #7748 (search for "bad link").

Actually, after seeing it happen just now when running
test-documentation on my laptop, on some other link, it occurs to me
that I've seen this before -- it's fairly common in Travis, too.  It's
just that it doesn't actually cause the build to fail :-/, and on
Travis we haven't been paying as close attention to slow builds as we
are on Circle right now.

tools/test-documentation

@@ -15,6 +15,9 @@ case $1 in
     -L|--loglevel)
         loglevel="$1 $2"
         ;;
+    --skip-check-links)
+        skip_check_links=1
+        ;;
 esac
 
 cd "$(dirname "$0")"/../docs
@@ -26,6 +29,11 @@ rm -rf _build
 # The crawler would take a very long time to finish and TravisCI would fail as a result.
 sphinx-build -j8 -b html -d _build/doctrees -D html_theme_options.collapse_navigation=True . _build/html
 
+if [ -n "$skip_check_links" ]; then
+    color_message 94 "Skipped testing links in documentation."
+    exit 0
+fi
+
 color_message 94 "Testing links in documentation..."
 
 cd ../tools/documentation_crawler

tools/travis/backend

@@ -19,7 +19,7 @@ set -x
 
 ./tools/test-migrations
 ./tools/optimize-svg
-./tools/test-documentation
+./tools/test-documentation ${CIRCLECI:+--skip-check-links}
 ./tools/test-help-documentation
 ./tools/test-api
 ./tools/test-locked-requirements