MyFirstContribution: add standalone section on cover letter

MyFirstContribution: add standalone section on cover letter

An explanation of the purpose of the cover letter is included in the
"Sending Patches with git send-email" / "Preparing Email" section but is
missing from the "Sending Patches via GitGitGadget" section.

Add a standalone section "The cover letter" under the "Getting Started:
Anatomy of a Patch Series" header to explain what the cover letter is
used for and to draft the cover letter of the 'psuh' topic used in the
tutorial.

For now we mostly copy content from the "Sending Patches with git
send-email" section but do not adjust that section, nor the GGG section,
to reference the new section. This is done in following commits.

Also, adjust the "Preparing Email" Asciidoc anchor to avoid conflicts.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>