Document absolute factorization and absolute irreducibility over QQbar

[daksha607]

This PR documents SageMath’s existing support for absolute factorization and absolute irreducibility of polynomials via factorization over QQbar and AA.

Although Sage already supports factorization over the algebraic closure of the rationals (using Singular’s absfact library for multivariate polynomials), this functionality was not explicitly documented or linked to the standard terminology used in the literature. As a result, it was difficult for users to discover via the documentation or search engines.

This change adds a dedicated section to the polynomial rings reference, including clear explanations and examples demonstrating absolute factorization and absolute irreducibility testing.

Fixes #41484.

📝 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

None.