Add documentation of rspec artifact collectors

What does this MR do and why?

Adds the rspec artifact collector jobs to: https://docs.gitlab.com/ee/development/pipelines/#backend-pipeline

Screenshots or screen recordings

Preview here: https://gitlab.com/gitlab-org/gitlab/-/blob/9084bbe9f258d91bd746e7af63670e8e3d5adcf8/doc/development/pipelines/index.md#backend-pipeline

How to set up and validate locally

N/A

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.