Update Capybara docs with RSpec example

[sliminas]

This adds an example how to configure RSpec with phlex-testing-capybara.

It also

• mentions to include Phlex::Testing::Rails::ViewHelper just as in the Nokogiri docs, which was missing before
• updates the test name TestExample to HelloExample to match the name of the tested Hello class

[sliminas]

What do you think? @joeldrapper

[joeldrapper]

This is out of date now, but I’m keeping it open so we can copy it into the new Vitepress doc site currently in main.