(POC) Add monthly benchmarking results to the changelog

[amyblais]

We're doing a great job benchmarking our releases and the test results are useful as they show consistent improvement. Proposal is to make the test results more visible for customer trust.

[github-actions]

Newest code from mattermost has been published to preview environment for Git SHA 47d6ac9

[agarciamontoro]

This is great, @amyblais, thank you!

I know I'm not consistent in my wording in the reports, but now that we're adding it to our more visible docs, it makes sense to agree on a convention. I like the wording of the three last reports in the PR. Tweaking a couple of things to be more specific, we could use the following template:

Benchmarking test results showed no significant difference: a x% increase/decrease in the number of supported users for the new release, which lies within the [-5%, +5%] prediction interval. View the full raw data and methodology in our Performance Reports repository.

I'm open to tweaking that, but consistency is important. Also, I think that starting with "results showed no significant difference" is good to deliver the most important information first.

I'll also adapt my future reports to the template decided here.