docs: mention MyFirstContribution in more places

While the MyFirstContribution guide exists and has received some use and
positive reviews, it is still not as discoverable as it could be. Add a
reference to it from the GitHub pull request template, where many
brand-new contributors may look. Also add a reference to it in
SubmittingPatches, which is the central source of guidance for patch
contribution.

Signed-off-by: Emily Shaffer <emilyshaffer@google.com>
Reviewed-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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