Addition of Kähler algebras as a category

[25shriya]

This PR adds Kähler algebras as a category KahlerAlgebras as defined in Theorem 2.28 of this paper. It changes the category of Chow rings to KahlerAlgebras() & CommutativeRings().Quotients(). A method, lefschetz_element() is also implemented for Chow rings which returns a Lefschetz element as provided in Corollary 2.30 of the paper above, which is used for the Hodge-Riemann relations computation.

@tscrim

📝 Checklist

• The title is concise and informative.
• The description explains in detail what this PR is about.
• I have linked a relevant issue or discussion.
• I have created tests covering the changes.
• I have updated the documentation and checked the documentation preview.

⌛ Dependencies

[github-actions]

Documentation preview for this PR (built with commit 0448df5; changes) is ready! 🎉
This preview will update shortly after each push to this PR.