doc/dev: reorganize flakes documentation
Created by: bobheadxi
Consolidates documentation about flakes processes and trims down the principles page to just principles. Some shuffling as I figure out things to add for https://github.com/sourcegraph/sourcegraph/issues/29889
Also removes references to Distribution from the process, and updates the flakes issue template to be more structured based on https://docs.github.com/en/communities/using-templates-to-encourage-useful-issues-and-pull-requests/syntax-for-issue-forms - can't test this until merge though