Add explicit inclusion of the wider community in function of the MR Review SLO time

What does this MR do and why?

This MR makes it explicit that community MRs should be treated as equal towards our review time SLO (https://docs.gitlab.com/ee/development/code_review.html#review-response-slo). This is to avoid confusion that is present in gitlab-com/www-gitlab-com#13718 (closed).

Questions I still have

Perhaps this is not the right place to make that explicit. Perhaps it needs to go even higher up in the chain. Having it here does make it very clear what our goals are towards the wider community.

Follow up

There is another question what the SLO should be for follow up reviews. Eg, the wider community author responds to the review and thinks it is ready again, should the same SLO go in effect? Eg, should the reviewer respond within 2 working days? I don't necessarily want to start that discussion here so once we have this nailed down, I'll open an MR for this follow-up to make that more clear.