Give ADR-020 the docmeta block every page carries

The full suite caught it: docs/INDEX.md routes by this metadata, so a page
without it is invisible to the router that exists to make the corpus findable.
Failed on gh-main and would have failed CI.

docs/adr/ADR-020-worker-self-recycle-on-plugin-change.md

@@ -1,3 +1,11 @@
+<!-- docmeta
+purpose: explanation
+audience: developer
+status: current
+summary: Why enabling a plugin no longer needs a manual restart, and where it still does.
+loadwith: adr/ADR-013-plugin-catalog-and-lean-builds.md
+-->
+
 # ADR-020: Worker self-recycle when the enabled-plugin set changes
 
 Status: Accepted