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

dockers/texcmp/README.md

@@ -30,7 +30,7 @@ the even larger overhead of setting up docker and creating the initial
 image, then you may instead execute the commands
 
     cd dockers/texcmp
-    yarn install
+    yarn
     node texcmp.js
 
 from the root of your KaTeX directory tree.  Required tools include the