Introduce REUSE compliance

This commit introduces REUSE compliance by annotating all files
with SPDX information and placing the reused licences in the
LICENSES folder.

We additionally removed the docheader tool which is made obsolete
by this change.

The main LICENSE and copyright text of the project is now not under
my personal name anymore, and it belongs to "The Respect Project
Contributors" instead.

This change restores author names to several files, giving the
appropriate attribution for contributions.

.github/workflows/continuous-integration-code.yml

@@ -84,9 +84,6 @@ jobs:
       - name: Install dependencies
         run: composer install --prefer-dist
 
-      - name: Run DocHeader
-        run: vendor/bin/docheader check library/ tests/
-
       - name: Run PHP_CodeSniffer
         run: vendor/bin/phpcs
 

.github/workflows/reuse.yml

@@ -0,0 +1,28 @@
+name: REUSE
+
+on:
+  push:
+    branches:
+      - main
+  pull_request:
+    branches:
+      - '*'
+
+jobs:
+  reuse-check:
+    name: Compliance Check
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v6
+
+      - name: Set up Python
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.x'
+
+      - name: Install REUSE tool
+        run: pip install reuse
+
+      - name: Run REUSE check
+        run: reuse lint