Fix CI markdown link check http 500 errors (HDFGroup#4556)

Sites like GitLab can have internal problems that return http 500
errors while they fix their problems. Some sites also return http
200 OK, which is fine.

This PR adds a config file to the markdown link check so those
are considered "passing" and don't break the CI.

.github/workflows/markdown-link-check.yml

@@ -6,9 +6,13 @@ on:
   pull_request:
     branches: [ develop ]
 
+# The config file handles things like http 500 errors from sites like GitLab
+# and http 200 responses
 jobs:
   markdown-link-check:
     runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@master
     - uses: gaurav-nelson/github-action-markdown-link-check@v1
+      with:
+        config-file: '.github/workflows/markdown_config.json'

.github/workflows/markdown_config.json

@@ -0,0 +1,3 @@
+{
+    "aliveStatusCodes": [200, 500]
+}