mathlib3

2c6df07f - chore(model_theory/*): Split up big model theory files (#12918)

chore(model_theory/*): Split up big model theory files (#12918) Splits up `model_theory/basic` into `model_theory/basic`, `model_theory/language_maps`, and `model_theory/bundled`. Splits up `model_theory/terms_and_formulas` into `model_theory/syntax`, `model_theory/semantics`, and `model_theory/satisfiability`. Adds to the module docs of these files.