docs: Update "How to test parcels and preview scenes" section

[sandrade-dcl]

What?

Fix decentraland/unity-renderer/issues/551

Update the "How to test parcels and preview scenes" section.

Why?

The processes described in the documentation were out of date.

[github-actions[bot]]

After the CI passes:

• This branch can be previewed at https://play.decentraland.zone/branch/docs/update-how-to-test-parcels-and-preview-scenes/index.html
• The decentraland-ecs package can be tested in scenes by running

  npm install "https://play.decentraland.zone/branch/docs/update-how-to-test-parcels-and-preview-scenes/decentraland-ecs.tgz"