Redis survival guide for SREs

Craig Miskellrequested to merge
redis-survival-guide into master

To try and capture and share some information learned by the Scalability team during redis investigations (and some ancillary info). Partially handles content requested in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/10400 (though that needs more) and in pursuit of the first item in gitlab-com/gl-infra/scalability#438

Edited by Craig Miskell

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