Fixes #33083

Add a note on using `yarn autoclean`.

Link to W3C reference was broken, this uses the latest URL, along with HTTPS.

Make various wording tweaks to cater to users who are viewing the README
on NPM. Notably, don't highlight Yarn specifically in the installation
instructions -- even though this is the preferred tool of choice
especially in the Ruby community, some people still use NPM (and,
really, ES2015+ syntax has nothing to do with NPM or Yarn).

[ci skip]

[ci skip]

We are going to make rails/rails the official repository