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/.

.gitea/workflows/build-template.yml

@@ -1,29 +0,0 @@
-# Example build.yml for a libreshop component repo.
-# Copy to .gitea/workflows/build.yml and remove the "on:" override below.
-#
-# Replace the existing multi-step build.yml with these ~20 lines.
-
-name: build
-
-on:
-  push:
-    branches: [main]
-    tags: ["v*"]
-  pull_request:
-    branches: [main]
-
-jobs:
-  build:
-    runs-on: ubuntu-latest
-    container:
-      image: git.librete.ch/public/runner-image:v0.2.0@sha256:f60c587d3c0b0aac04a572db5349e27672bf76baec2ce547a3dcc28cebcf1b7e
-    timeout-minutes: 20
-    steps:
-      - uses: actions/checkout@v4
-
-      - uses: https://git.librete.ch/public/actions/.gitea/actions/docker-build@main
-        with:
-          registry: ${{ secrets.REGISTRY }}
-          registry_user: ${{ secrets.REGISTRY_USER }}
-          registry_pass: ${{ secrets.REGISTRY_PASS }}
-          publish: ${{ github.event_name == 'push' && vars.PUBLISH_ENABLED == 'true' }}