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/content/advanced/backend/upgrade-guides/1-3-8.md

@@ -4,7 +4,7 @@ Updating your medusa server to version `1.3.8` may cause issues when using NPM.
 
 ## Update Using Yarn
 
-We highly recommend using [yarn](https://yarnpkg.com/) when working with Medusa. Updating with yarn should resolve any issues you might run into during the update.
+It's highly recommended to use [yarn](https://yarnpkg.com/) when working with Medusa. Updating with yarn should resolve any issues you might run into during the update.
 
 ## Resolving Update Issues with NPM