User guide: map from "Equality constraints" section to relevant LANGUAGE extensions

It would be great if the section "Equality constraints" contained links to the LANGUAGE pragmas that enable equality constraints.

• Rendered: https://ghc.gitlab.haskell.org/ghc/doc/users_guide/exts/equality_constraints.html#equality-constraints
• Source: https://gitlab.haskell.org/ghc/ghc/-/blob/9408b086775960a2e0a6b0b6f8091e79bcf9ddd5/docs/users_guide/exts/equality_constraints.rst