Revert to using Yarn for now

A switch to npm (or Yarn 3 or pnpm) would all be good moves, but will
also require updating the CI config. For the sake of being able to make
consistent progress on upgrades, simply keep using Yarn for this piece.

Author: chriskrycho

README.md

@@ -42,7 +42,7 @@ locally yourself.
 ```
 git clone https://github.com/ember-learn/ember-api-docs.git
 cd ember-api-docs
-npm install
+yarn install
 ember serve
 ```
 View at http://localhost:4200
@@ -53,9 +53,9 @@ To run a11y tests, run `test_a11y=yes ember serve`
 
 ## Linting
 
-* `npm run lint:hbs`
-* `npm run lint:js`
-* `npm run lint:js -- --fix`
+* `yarn run lint:hbs`
+* `yarn run lint:js`
+* `yarn run lint:js -- --fix`
 
 ## Staging and Deployment