ENH: Add link checker GHA workflow file

Add link checker GHA workflow file: use the `lychee` action to check for
broken links in Markdown, HTML, reStructuredText, and other files.

Repository: https://github.com/lycheeverse/lychee-action
Documentation: https://lychee.cli.rs/introduction/

Author: jhlegarreta

.github/workflows/linkcheck.yml

@@ -0,0 +1,19 @@
+name: Check links
+
+on: [push, pull_request]
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  linkcheck:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check links
+        id: lychee
+        uses: lycheeverse/lychee-action@v2
+        with:
+          fail: true

.lychee.toml

@@ -0,0 +1,11 @@
+# Exclude patterns for links that are expected to fail in CI:
+# - Jinja2/Liquid template expressions in filenames
+# - insight-journal.org: historical journal now offline
+# - gnuplot.info: site exists but has TLS issues intermittently
+exclude = [
+  "%7B%7B",                            # URL-encoded {{ in Jinja template file paths
+  "insight-journal.org",               # Historical journal, offline
+  "www.gnuplot.info",                  # TLS certificate issues
+  "cmake-w3-externaldata-upload.on.fleek.co", # Web3 upload service, returns 403
+  "npmjs.com/package/@web3-storage",    # w3cli package, returns 403
+]

Documentation/Build/index.rst

@@ -242,7 +242,7 @@ VV
 .. _ImageViewer:           https://github.com/KitwareMedical/ImageViewer
 .. _ITKApps:               https://github.com/InsightSoftwareConsortium/ITKApps
 .. _ITK-SNAP:              http://www.itksnap.org/pmwiki/pmwiki.php
-.. _MITK:                  https://www.mitk.org/wiki/MITK
+.. _MITK:                  https://www.mitk.org/
 .. _Paraview:              https://www.paraview.org/
 .. _Qt:                    https://www.qt.io/developers/
 .. _VTK:                   https://vtk.org/