docs: add DCO sign-off instructions in Contributing guide

[Flashl3opard]

Description

Adds a section explaining how to include a DCO sign-off (Signed-off-by) in commits.

Motivation

While contributing, I noticed that commit sign-off (DCO) is required but not mentioned in the contributing guidelines. This can lead to confusion and rejected pull requests for new contributors, and may also increase the review overhead for maintainers.

This change adds clear instructions to help contributors follow the expected workflow.

Changes

• Added "Commit Sign-off (DCO)" section under Guidelines
• Included command usage (git commit -s)
• Added a quick way to verify sign-off

Checklist

• Documentation update only
• No code changes

Note: (I'm not an Outreachy Contributor ✌🏻)

[Copilot]

Pull request overview

Adds Developer Certificate of Origin (DCO) commit sign-off guidance to the contributing documentation to reduce contributor confusion and PR rejections.

Changes:

• Adds a “Commit Sign-off (DCO)” section describing how to create signed-off commits.
• Updates inline code formatting in the Tests section (e.g., tox, # no-qa).

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[sdglitched]

Kindly check the commit and make the suggested changes.

Also, for this small change only one commit should suffice. Please squash the multiple commits into a single one.

[sdglitched]

@Flashl3opard why are you trying to imply that you had unnecessary changes in the older commit?

As requested earlier, please squash the multiple commits into a single one and make sure the character count does not cross 50.

[Flashl3opard]

@sdglitched Thanks for the clarification. I’ve squashed the commits into a single commit. Let me know if anything else needs to be updated.