Add woodpecker integration for building and testing the EPUB

Signed-off-by: David Runge <dave@sleepmap.de>

.woodpecker/epub.yml

@@ -0,0 +1,24 @@
+# SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
+# SPDX-License-Identifier: CC0-1.0
+
+clone:
+  git:
+    image: woodpeckerci/plugin-git
+    settings:
+      lfs: false
+
+when:
+  branch: draft
+  event:
+    - push
+    - pull_request
+
+steps:
+  epub-check:
+    image: archlinux:latest
+    commands:
+      - pacman -Sy --needed --noconfirm archlinux-keyring
+      - pacman -Syu --needed --noconfirm epubcheck make patch python-myst-parser python-sphinx
+      # fix sphinx: https://github.com/sphinx-doc/sphinx/issues/11598
+      - patch -Np1 -d /usr/lib/python3.11/site-packages/ -i "$(pwd)/book/patches/sphinx-11766.patch"
+      - make -C book epub-check