List the Active Support Instrumentation guide in the index (as WIP)
Fix a few typos [ci skip]
Update grammar [ci skip]
Merge pull request #18190 from ef718/master

Fix grammar in Caching with Rails docs
Correct grammar to fix #18182 [ci skip]
Merge pull request #18185 from ivalkeen/add_missing_guides

List the Active Support Instrumentation guide in the index (as WIP)
Check for Rails.env instead of Rails
`Rails` constant is added by rails-html-sanitizer leading to bugs in
non-Rails apps using ActiveRecord and ActionMailer
Merge pull request #18196 from PavelPenkov/rails_env_instead_of_rails

Check for Rails.env instead of Rails
Add example to get Errored test to pass in Testing guide [ci skip]