runtime(doc): remove outdated Contribution section in pi_tutor

runtime(doc): remove outdated Contribution section in pi_tutor

Problem:  The Github repo link in the Contribution section has been
          archived for 5 years. So people who want to contribute to the
          tutor plugin should just send PR to Vim repo, similar to most
          other Vim features, so there is no need for a Contribution
          section in the plugin doc.

Solution: Replace it with an Original Author note at the beginning of
          the help document.

closes: #17341

Signed-off-by: Phạm Bình An <111893501+brianhuster@users.noreply.github.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>