Link bindep documentation in the infra index

This adds a link to the bindep documentation, but should not be
approved until such documentation is successfully published by the
parent change so as to avoid creating a broken hyperlink in the
meantime.

Change-Id: If1e65725d2134ba1b299ab89a729f1638e529e82

docs-site/infra-documents.yaml

@@ -1,4 +1,6 @@
 documents:
+  - name: Bindep
+    directory: bindep
   - name: Elastic Recheck
     directory: elastic-recheck
   - name: Git Review