Add blank symbol and link-panel pages (#18)

docs/PORTING.md

@@ -70,6 +70,7 @@ not interpret symbols into Age generation, which remains Milestone 5 work.
 ## Provenance rules
 
 - New code uses the `net.dnorberg.rivencraft` package.
+- Reused legacy art is recorded in [ASSET_PROVENANCE.md](ASSET_PROVENANCE.md).
 - Adapted Mystcraft code retains its upstream copyright notice and is tracked
   in a nearby source comment or commit.
 - Generated, binary, and third-party dependency content is not copied from the