docs: remove included/readme image generation instructions

[MikeMcC399]

Issue

The included/README > Building and testing section includes outdated instructions to build a new image using the removed script add:included. The instructions appear to be targeted to contributors, as they describe pushing to Cypress' Docker Hub.

Background

• PR https://github.com/cypress-io/cypress-docker-images/pull/812 replaced the process to generate new images, including updating the CONTRIBUTING document to include instructions on generating all types of images.
• PR https://github.com/cypress-io/cypress-docker-images/pull/1055 removed the obsolete script definitions.

Change

Completely remove the outdated and misplaced section included/README > Building and testing.

[cypress-app-bot]

• Create a Draft Pull Request if your PR is not ready for review. Mark the PR as Ready for Review when you're ready for a Cypress team member to review the PR.

[MikeMcC399]

Preview

• included/README