fix(ci): move build-template.yml out of .gitea/workflows/

The file was a copy-paste template for libreshop component repos but
lived in .gitea/workflows/ — so Gitea auto-ran it against this repo
on every push, failing because there is no Dockerfile here. Every
push to main has been red since the template was committed.

Move to examples/ and document why it cannot live in .gitea/workflows/.

examples/build-template.yml

@@ -1,7 +1,9 @@
 # Example build.yml for a libreshop component repo.
-# Copy to .gitea/workflows/build.yml and remove the "on:" override below.
+# Copy to .gitea/workflows/build.yml in the consuming repo.
 #
-# Replace the existing multi-step build.yml with these ~20 lines.
+# Lives outside .gitea/workflows/ on purpose — Gitea would auto-run it
+# here against this repo (which has no Dockerfile) and the build would
+# fail every time.
 
 name: build