Add a very lightweight AI usage policy #1871

Originally discussed in #1630 , there might be some value in creating a lightweight AI usage policy. If we go this route, it can be linked to from within the PR checklist section.

There are many good examples online already, and the original discussion seemed to lean towards a lightweight beginning to this, e.g. something similar to or much lighter weight than what xarray did, but it can be anything else that is a ligthweight start that can be improved over time.

Example: xarray

xarray added an AI usage policy a few months ago (pydata/xarray#11126), which is linked from their PR form as follows:

### AI Disclosure

<!--- Please review our AI & contribution guidelines: https://docs.xarray.dev/en/stable/contribute/ai-policy.html.
    Remove this section if your PR does not contain AI-generated content. --->

- [ ] This PR contains AI-generated content.
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