Update the contributing guide for Docusaurus (#7327)

docs/docs/developers/contributing.md

@@ -39,12 +39,20 @@ The following commands are now available from the repository root:
 > gulp lint                 // perform code linting (ESLint)
 > gulp test                 // perform code linting and run unit tests
 > gulp test --browsers ...  // test with specified browsers (comma-separated)
-> gulp docs                 // build the documentation in ./dist/docs
-> gulp docs --watch         // starts the gitbook live reloaded server
 ```
 
 More information can be found in [gulpfile.js](https://github.com/chartjs/Chart.js/blob/master/gulpfile.js).
 
+### Documentation
+
+We use [Docusaurus v2](https://v2.docusaurus.io/docs/introduction) to manage the docs which are contained as Markdown files in the docs directory. You can run the doc server locally using the commands provided by Docusaurus:
+
+```
+$ cd docs
+$ npm install
+$ npm run start
+```
+
 ### Image-Based Tests
 
 Some display-related functionality is difficult to test via typical Jasmine units. For this reason, we introduced image-based tests ([#3988](https://github.com/chartjs/Chart.js/pull/3988) and [#5777](https://github.com/chartjs/Chart.js/pull/5777)) to assert that a chart is drawn pixel-for-pixel matching an expected image.