System Spec feature: include System Tests text

[olleolleolle]

This PR introduces a few sentences from the Rails documentation on System Testing.

Goals:

1. keep more of the story in the same document - easier to navigate
2. tell the story in an rspec-rails frame - talk about how to get things done in this spec

When we close in on those goals, perhaps the link to the System Tests documentation can be de-emphasised.

Questions for reviewer

• What is the smallest iterative step we can take towards those goals?
• Should this PR include more Scenarios?

[fables-tales]

@olleolleolle I think it might be good to document that this runs your tests in a transaction, and so unlike feature specs with javascript, you don't need database cleaner.

[fables-tales]

I'm happy to merge this as is. I'm the wrong person to write the documentation here though, as I have a very detailed understanding of how this works, and thererfore don't know what isn't obvious. @olleolleolle do you think anything else is missing from our docs?

[olleolleolle]

@samphippen I'll just fix a typo, and add a note that Capybara's auto-required for a system spec.

[olleolleolle]

@samphippen There.

[JonRowe]

Good improvement overall, a few notes.

[olleolleolle]

I changed wordings.

Thanks for caring and giving such actionable feedback on this PR.

This is good progress for a short sitting, I'm pleased.