Add link to external documentation of rspec-rails feature specs

What does this MR do?

This MR updates the Testing Standards and Style Guidelines document by adding a link to the (official) rspec-rails feature specs documentation.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

Prior to working on this project, most of my experience in rails projects has been with minitest as the test framework. I had used rspec, but only outside of rails contexts (I hadn't used rspec-rails), so at first I didn't realize that "rspec feature specs" was an external concept (formalized in rspec-rails). I thought adding this link to the docs could make this more immediately obvious.

Since this is just a documentation change, I added [ci skip] in the commit message to skip the builds