doc(NumberTheory/Padics/MahlerBasis): add credit line (#19845)

The file `NumberTheory/Padics/MahlerBasis.lean` is largely based on the ETH Zürich bachelor thesis of Giulio Caflisch. This PR gives Giulio due credit for this -- he is already included in the "Authors" line, but this does not appear in the online reference manual. I am following here the example of [JacobiSum/Basic](https://leanprover-community.github.io/mathlib4_docs/Mathlib/NumberTheory/JacobiSum/Basic.html), which is likewise based on a bachelor thesis project.

Mathlib/NumberTheory/Padics/MahlerBasis.lean

@@ -25,6 +25,9 @@ converges (uniformly) to `f`, and this construction defines a Banach-space isomo
 
 For this, we follow the argument of Bojanić [bojanic74].
 
+The formalisation of Mahler's theorem presented here is based on code written by Giulio Caflisch
+for his bachelor's thesis at ETH Zürich.
+
 ## References
 
 * [R. Bojanić, *A simple proof of Mahler's theorem on approximation of continuous functions of a