Check external links once a month

.github/workflows/check-links.yaml

@@ -0,0 +1,20 @@
+name: Check external href links in the documentation
+
+on:
+  schedule:
+    # Run on the first of each month at 9:00 AM (See https://pubs.opengroup.org/onlinepubs/9699919799/utilities/crontab.html#tag_20_25_07)
+    - cron: "0 9 1 * *"
+
+jobs:
+  markdown-link-check:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@master
+      - name: Prepare markdown-link-check
+        run: mkdir docs/ROOT && cp CHANGELOG.md README.md docs/ROOT
+      - uses: gaurav-nelson/github-action-markdown-link-check@v1
+        with:
+          use-quiet-mode: 'yes'
+          use-verbose-mode: 'no'
+          config-file: 'mlc_config.json'
+          folder-path: 'docs/'

.github/workflows/tests.yml

@@ -44,15 +44,6 @@ jobs:
         with:
           args: CHANGELOG.md CONTRIBUTING.md README.md docs/
           config: '.markdownlint.yml'
-      - name: Prepare markdown-link-check
-        run: mkdir docs/ROOT && cp CHANGELOG.md README.md docs/ROOT
-      - name: Run markdown-link-check
-        uses: gaurav-nelson/github-action-markdown-link-check@v1
-        with:
-          use-quiet-mode: 'yes'
-          use-verbose-mode: 'no'
-          config-file: 'mlc_config.json'
-          folder-path: 'docs/'
   tests:
     name: Unit tests
     runs-on: ubuntu-latest

docs/adr/0011-test-external-links-in-documentation.md

@@ -0,0 +1,47 @@
+# Test external links in documentation
+
+## Context and Problem Statement
+
+The JabRef repository contains Markdown (`.md`) files documenting the JabRef code.
+The documentation contains links to external resources.
+For high-quality documentation, external links should be working.
+
+## Decision Drivers
+
+* Checking external links should not cause issues in the normal workflow
+
+## Considered Options
+
+* Check external links once a month
+* Check external links in the "checkstyle" task
+* Do not check external links
+
+## Decision Outcome
+
+Chosen option: "\[option 1\]", because \[justification. e.g., only option, which meets k.o. criterion decision driver \| which resolves force force \| … \| comes out best \(see below\)\].
+
+### Positive Consequences
+
+* Automatic notification of broken external links
+
+### Negative Consequences
+
+* Some external sites need to [be disabled](https://github.com/JabRef/jabref/pull/6542/files). For isntance, GitHub.com always returns "forbidden". A [filter for status is future work of the used tool](https://github.com/tcort/markdown-link-check/issues/94#issuecomment-634947466).
+
+## Pros and Cons of the Options
+
+### Check external links once a month
+
+* Good, because does not interfer with the normal development workflow
+* Bad, because an additional workflow is required
+
+### Check external links in the "checkstyle" task
+
+* Good, because no seperate workflow is required
+* Bad, because checks fail independent of the PR (because external web sites can go down and go up independent of a PR)
+
+### Do not check external links
+
+* Good, because no testing at all is required
+* Bad, because external links break without any notice
+* Bad, because external links have to be checked manually