Warn on use of capture shortcode

If anyone uses the capture shortcode, emit a warning. The website
no longer uses capture shortcodes.

layouts/shortcodes/capture.html

@@ -3,5 +3,6 @@
 {{- if not $id -}}
 {{- errorf "missing id in capture" -}}
 {{- end -}}
-{{- $capture_id := printf "__cid_%s" $id -}}
+{{- $capture_id := printf "capture %s" $id -}}
 {{- .Page.Scratch.Set $capture_id .Inner -}}
+{{ warnf "Invalid shortcode: %s, in %q" $capture_id (relLangURL .Page.Path) }}