Merge branch 'ds/doc-ggg-pr-fork-clarify' into maint-2.51

Update the instruction to use of GGG in the MyFirstContribution document to say that a GitHub PR could be made against `git/git` instead of `gitgitgadget/git`. * ds/doc-ggg-pr-fork-clarify: doc: clarify which remotes can be used with GitGitGadget