Add ability to run markdown benchmarks in CI

What does this MR do and why?

Create a manual CI job that can run the rake benchmark:banzai command to provide performance numbers for the Banzai pipeline.

Testing

When a pipeline is running, go to the "Stage: benchmark" jobs and click the play button next to benchmark-markdown

Once it's done, check the artifacts to see that benchmark-markdown.txt has been created and contains the results from the rake benchmark:banzai task.

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.

Relates to #416498 (closed)