Update icinga checks and docs

docs/20_Checks/_index.md

@@ -17,9 +17,11 @@ There is one include script used by all checks:
 * check_couchdb-lb
 * [check_cpu](check_cpu.md)
 * [check_cronstatus](check_cronstatus.md)
-* check_disk-io
-* check_dns_responsetime
+* [check_disk-io](check_disk-io.md)
+* [check_dns_responsetime](check_dns_responsetime.md)
 * [check_eol](check_eol.md)
+* [check_fs_errors](check_fs_errors.md)
+* [check_fs_writable](check_fs_writable.md)
 * check_haproxy_health
 * check_haproxy_status
 * check_memory