ci: Add steps to test for broken links using lychee

Call the new `html-linkcheck` target to fail on broken links.

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

.woodpecker/links.yml

@@ -0,0 +1,23 @@
+# SPDX-FileCopyrightText: 2023 The "Notes on OpenPGP" project
+# SPDX-License-Identifier: CC0-1.0
+
+clone:
+  git:
+    image: woodpeckerci/plugin-git
+    settings:
+      partial: true
+      lfs: false
+
+when:
+  branch: draft
+  event:
+    - push
+    - pull_request
+
+steps:
+  link-check:
+    image: archlinux:latest
+    commands:
+      - pacman -Sy --needed --noconfirm archlinux-keyring
+      - pacman -Syu --needed --noconfirm lychee make python-myst-parser python-sphinx
+      - make -C book html-linkcheck