Replace yarn run and yarn install with yarn, remove building with npm instructions (#1528)

* Replace `yarn run` with `yarn`

* Replace `yarn install` with `yarn`

* Remove building using `npm` from node.md

.circleci/config.yml

@@ -17,7 +17,7 @@ restore_node_modules_cache: &restore_node_modules_cache
 yarn_install: &yarn_install
   run:
     name: Install dependencies
-    command: yarn install
+    command: yarn
 
 screenshotter: &screenshotter
   steps:
@@ -71,7 +71,7 @@ jobs:
 
       - run:
           name: Build KaTeX
-          command: yarn run build
+          command: yarn build
 
       - store_artifacts:
           path: dist/katex.js