Update Rails Docs and Product Onboarding

[stephanie-anderson]

There is an existing improvement for Rails onboarding, it should be reflected in the Docs and Onboarding

https://docs.sentry.io/platforms/ruby/guides/rails/

1. Existing install feedback

this did not work for me, I had to use gem install sentry-rails instead.

2. An install command step should be add, small QoL thing

bundle install

3. Creating config file

Initialize the SDK within your config/initializers/sentry.rb:

Should at least partially be replaced by

Rails users will be able to use bin/rails generate sentry to generate their config/initializers/sentry.rb file.