doc: add contributing detail for git Signed-off-by trailer

[MikeMcC399]

Fixes: #64799

Situation

The instructions under contributing > Pull requests > Commit message guidelines document section are formulated without reference to the git commit -s option which provides for assisted addition of the required Signed-off-by trailer:

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6. Your commit must contain the Signed-off-by line with your name and email
   address as an acknowledgement that you agree to the Developer Certificate of Origin.
   Bot generated commits are exempt from this requirement. If a commit has
   multiple authors, the Signed-off-by line should be added for each author;
   and at least one should match the author information in the commit metadata.
   This rule does not apply to dependency updates (e.g. cherry-picks), release
   commits, or backport commits.

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The examples of