[Chore] Remove doc from published react-admin package

[djhi]

Problem

We currently include a copy of the docs directory in the published react-admin package. This makes it quite heavy and is probably useless.

Solution

• remove the command that copy the docs directory when building the package.
• remove the docs directory from the included files of the package.json for react-admin

Additional Checks

• The PR targets master for a bugfix, or next for a feature

[fzaninotto]

You should probably keep this one for those of us who have an old copy of the docs