[css-gcpm] Add TR metadata, and shift title into Title metadata.

Author: tabatkins

css-gcpm/Overview.bs

@@ -1,11 +1,12 @@
-<h1>CSS Generated Content for Paged Media Module</h1>
 <pre class='metadata'>
+Title: CSS Generated Content for Paged Media Module
 Status: ED
 Work Status: Exploring
 Shortname: css-gcpm
 Level: 3
 Group: csswg
 ED: https://drafts.csswg.org/css-gcpm/
+TR: https://www.w3.org/TR/css-gcpm-3/
 Editor: Dave Cramer, Hachette Livre, dave.cramer@hbgusa.com
 Former Editor: Håkon Wium Lie, Opera Software
 Link defaults: css-content-3 (property) counter-increment, css-content-3 (property) counter-reset, css21 (property) string, css21 (property) max-height