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/styles/docs/WritingLevel.yml

@@ -0,0 +1,9 @@
+extends: metric
+message: "Try to keep the Flesch-Kincaid grade level (%s) below 8."
+link: https://en.wikipedia.org/wiki/Flesch%E2%80%93Kincaid_readability_tests
+
+formula: |
+    (0.39 * (words / sentences)) + (11.8 * (syllables / words)) - 15.59
+
+condition: "> 8"
+level: suggestion