Update the executors (and documentation) to reference circleci/browser-tools for using headless Chrome for functional test suites

[jrgriffiniii]

[jrgriffiniii]

https://github.com/CircleCI-Public/cimg-ruby/blob/main/2.7/browsers/Dockerfile and https://github.com/CircleCI-Public/cimg-ruby/blob/main/2.7/node/Dockerfile provide the best practices supported by CircleCI

[jrgriffiniii]

https://github.com/samvera/samvera-circleci-orb/blob/main/src/executors/ruby.yml#L10 indicates that this may already have headless chrome installed.

[jrgriffiniii]

samvera/browse-everything#407 features an excellent example of this.