Actions: Ensure well-formed and valid XML

.github/workflows/valid-xml.yml

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+#                          __  __            _
+#                       ___\ \/ /_ __   __ _| |_
+#                      / _ \\  /| '_ \ / _` | __|
+#                     |  __//  \| |_) | (_| | |_
+#                      \___/_/\_\ .__/ \__,_|\__|
+#                               |_| XML parser
+#
+# Copyright (c) 2021 Sebastian Pipping <sebastian@pipping.org>
+# Licensed under the MIT license:
+#
+# Permission is  hereby granted,  free of charge,  to any  person obtaining
+# a  copy  of  this  software   and  associated  documentation  files  (the
+# "Software"),  to  deal in  the  Software  without restriction,  including
+# without  limitation the  rights  to use,  copy,  modify, merge,  publish,
+# distribute, sublicense, and/or sell copies of the Software, and to permit
+# persons  to whom  the Software  is  furnished to  do so,  subject to  the
+# following conditions:
+#
+# The above copyright  notice and this permission notice  shall be included
+# in all copies or substantial portions of the Software.
+#
+# THE  SOFTWARE  IS  PROVIDED  "AS  IS",  WITHOUT  WARRANTY  OF  ANY  KIND,
+# EXPRESS  OR IMPLIED,  INCLUDING  BUT  NOT LIMITED  TO  THE WARRANTIES  OF
+# MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN
+# NO EVENT SHALL THE AUTHORS OR  COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM,
+# DAMAGES OR  OTHER LIABILITY, WHETHER  IN AN  ACTION OF CONTRACT,  TORT OR
+# OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE
+# USE OR OTHER DEALINGS IN THE SOFTWARE.
+
+name: Ensure well-formed and valid XML
+
+on:
+  pull_request:
+  push:
+  schedule:
+    - cron: '0 2 * * 5'  # Every Friday at 2am
+
+jobs:
+  checks:
+    name: Ensure well-formed and valid XML
+    runs-on: ubuntu-20.04
+    steps:
+    - uses: actions/checkout@v2.3.4
+
+    - name: Install build dependencies
+      run: |-
+        sudo apt-get install --yes --no-install-recommends -V \
+            docbook \
+            libxml2-utils \
+            w3c-sgml-lib
+
+    - name: Ensure well-formed and valid XML
+      run: |
+        set -x
+        xmllint --noout --valid expat/doc/reference.html
+        xmllint --noout --valid expat/doc/xmlwf.xml