sagemathgh-39495: Add documentation of Gabidulin codes in the reference manual
    
The file `sage.coding.gabidulin_code.py` was not included in the
reference manual.
This PR fixes this.

### 📝 Checklist

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- [x] The title is concise and informative.
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preview.
    
URL: sagemath#39495
Reported by: Xavier Caruso
Reviewer(s): Kwankyu Lee

Author: Release Manager

src/doc/en/reference/coding/index.rst

@@ -70,6 +70,7 @@ computations for structural invariants are available.
    sage/coding/goppa_code
    sage/coding/kasami_codes
    sage/coding/ag_code
+   sage/coding/gabidulin_code
 
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