Add option to check only internal links for MR

doc.zih.tu-dresden.de/util/check-links.sh

@@ -36,6 +36,7 @@ usage: $0 [file | -a | -c]
 Options:
   <file>          Check links in the provided markdown file
   -a; --all       Check links in all markdown files
+  -i; --interal   Check links in all markdown files (internal links only)
   -c; --changed   Check links in all git-changed files
 EOF
 }
@@ -101,7 +102,6 @@ function check_changed_files(){
 # Make use of the html-proofer-plugin and build documentation
 function check_all_files_hpp() {
   export ENABLED_HTMLPROOFER=true
-  export ENABLED_HTMLPROOFER_EXTERNAL_URLS=true
   echo "Info: Building documentation and checking internal and external links. Might take some time."
   # Switch for wikiscript
   if [[ -d "doc.zih.tu-dresden.de" ]]; then
@@ -118,6 +118,11 @@ if [[ $# -ge 1 ]]; then
     exit
   ;;
   -a | --all)
+    export ENABLED_HTMLPROOFER_EXTERNAL_URLS=true
+    check_all_files_hpp
+  ;;
+  -i | --internal)
+    export ENABLED_HTMLPROOFER_EXTERNAL_URLS=false
     check_all_files_hpp
   ;;
   -c | --changed)