Merge branch 'pb/ggg-in-mfc-doc'

Documentation update. * pb/ggg-in-mfc-doc: MyFirstContribution: drop PR description for GGG single-patch contributions MyFirstContribution: reference "The cover letter" in GitGitGadget section MyFirstContribution: reference "The cover letter" in "Preparing Email" MyFirstContribution: add standalone section on cover letter MyFirstContribution: add "Anatomy of a Patch Series" section