docs: integrate Vale for documentation linting (#2242)

* added value rules

* resolved errors raised by vale

* added github action

* fixes to github action

* added details in contribution guidelines

* added rule for numbers

* limited checks to errors

docs/get-files.sh

@@ -0,0 +1,20 @@
+#!/bin/bash
+
+list=""
+# get directories in content other than reference
+for i in `find content -type d -maxdepth 1 -not -path 'content/references' -not -path 'content'`
+do
+  if [ ${#list} -gt 0 ]; then
+    list+=','
+  fi
+  list+="\"docs/$i\""
+done
+#get files in content (not nested)
+for i in `find content -type f -maxdepth 1 -not -path 'content/references'`
+do
+  if [ ${#list} -gt 0 ]; then
+    list+=','
+  fi
+  list+="\"docs/$i\""
+done
+echo "::set-output name=LIST::[$list]"