Update community docs to recommend use of "Co-authored-by" in git commits

discussion on [DISCUSS] switch from "Ammending-Author" to "Co-authored-by" in commit messages seems to have out in favor.

Updated section should include a brief explanation (that includes "multiple authors" expressly instead of just the "fixed up this thing" that's there for Amending-Author). It should also have pointers to the github feature explanation. So long as those docs exist they're pretty good.