Docs: Add Vale test to provide readability score
Problem to solve
It would be helpful to provide feedback to content creators about documentation pages that are overly complex, based on established readability tests, as an additional evaluation mechanism.
Proposal
Add a Vale test to indicate when a page has a readability score of more than 13 based solely on the Flesch-Kincaid test. Results for pages above this readability level should be noted as suggestions, and as an encouragement to evaluate the page at some point, but not as a warning or blocker.
Why 13? Based on the Flesch-Kincaid test, this would indicate a US grade level of 13, which is the second year of college. It's an arbitrary level, which we can adjust over time. I believe, however, that it's a metric we can use in our efforts to make our pages more comprehensible to the majority of our readers.