revert(docs): partially revert 326a1d1862 (#5095)

We tried to switch from `yarn` to `npm` because `yarn` ignores lockfiles
but learned that we missed a few key things.

For now, we are reverting docs and a few other changes that suggested
using `npm` instead of `yarn` until we fully remove `yarn` from the
codebase.
t Please enter the commit message for your changes. Lines starting

docs/manifest.json

@@ -21,7 +21,7 @@
       "children": [
         {
           "title": "npm",
-          "description": "How to install code-server using npm",
+          "description": "How to install code-server using npm or yarn",
           "path": "./npm.md"
         },
         {