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