Add a reusable workflow to add a PR to a project and set the status

[rowleya]

---

name: ⭐ New Reusable Workflow
about: For submitting a new Reusable.
title: '[NEW]: '
labels: enhancement ⭐
type: Feature

• Is this fixing an issue? If so, please #ref its ID. This helps a lot! (Note that an issue is not required for submitting a new action.)
  Helps to fix UoMResearchIT/RSE-Peer-Support#53

• What do you think this workflow does? We can see what it actually does by reading the code,
  but it's very helpful to have a short description of what you intend it to do.
  This workflow is used for adding any PRs where the reviewer is set to a Project of your chosing. This could be used to
  add PRs to the project for the repo, or to a separate repo where all PRs are being tracked.

• Have you considered whether there is an existing action by some other reasonably-trusted party to do this?
  This is based on code provided by GitHub as demo code but not seemingly actually made into an action anywhere!

• If you can, @ mention someone to review the code at least in the first instance.

• Do you have new bash scripts in your PR? Did you remember to mark them as executable?
  Not required

• Don't forget to give your action a README.md.
  Not an action. There is some documentation at the top though.

• Link to your action folder from the main README.md.
  Not an action, but added.

• Your action descriptor (action.yml) should include
  Not an action, but there are some docs in there.

• Consider whether your action interacts with itself and other actions.
  No interaction.

• It helps a lot if you can link to a build log that shows that your action works.
  As a reusable workflow that works on PRs this is tricky... but I did copy this from the SpiNNaker repo where we have started to use it, and it works there e.g.:
  https://github.com/SpiNNakerManchester/sPyNNaker/actions/runs/16069343414/job/45350280226

[dkfellows]

Sorry for taking so long to review this.