[Action] DCO enabled on GSF GitHub

Developer Certificate of Origin^1 is being enabled at the Green Software Foundation: https://github.com/orgs/Green-Software-Foundation/discussions/5

If you primarily change markdown files and other documents through the web interface, you should notice minimal changes.

Software developers will need to sign their commits on the commandline with -s.

Once enabled, your project will be able to accept code and text via Pull Request, from people that are not at Green Software Foundation member organisations.

For more information on the impact, rationale, and to ask any questions, please see the discussion.

[Willmish]

FYI: @bderusha @pritipath @gfmatthews @juzuluag @JenMadiedo

The current PRs (ones which did not go through DCO check) will go through, and we can overwrite this manually and approve the sign-offs on the check, but for future PRs it will be smoother if all commits are signed off (-s flag set in git commit)

[Sealjay]

If you have to modify some commits to add the sign-offs, an easy way is to use

* `git rebase -i <one commit before the first one you want to sign-off>`,

* change every action to `edit` instead of the default `pick`,

* then, for every commit, run
  
  * `git commit --amend -s`
  * close the editor with ctrl-x
  * `git rebase --continue`

That should do it!

Thanks @FabioTurati-NTT ! Quoted you here: https://github.com/orgs/Green-Software-Foundation/discussions/5#discussioncomment-5336378