Add AI contributor policy

[fendor]

We currently don't have any issues with too many AI contributed patches, so there is no immediate need to have a policy.

However, I think having an official policy would be good to avoid confusion in the future.
Since this project is closely related to GHC, it makes sense to me to adopt the GHC AI policy: https://gitlab.haskell.org/ghc/ghc/-/wikis/contributing/AI

[sgillespie]

I think it is a quite reasonable policy. Is there any way you think it should diverge?

[fendor]

Right now, I don't see a reason to diverge, so just adding the current AI policy to the HLS repo and maybe linking to GHC's sounds good to me.

[dschrempf]

Just my two cents:

• I think it should be a link, not a copy, so we have a single source of truth.
• I guess we should link from here: https://haskell-language-server.readthedocs.io/en/latest/contributing/index.html.