move pprpa.rst under pyscf-forge

Author: lijiachen417

source/user/index.md

@@ -36,7 +36,6 @@ agf2.rst
 mcscf.rst
 mcpdft.rst
 mrpt.rst
-pprpa.rst
 tddft.rst
 ```
 
@@ -54,17 +53,18 @@ lo.rst
 sgx.rst
 geomopt.rst
 gpu.rst
-```   
+```
 
 ```{toctree}
-:caption: PySCF-forge 
+:caption: PySCF-forge
 :maxdepth: 1
 
 pyscf-forge
+pprpa.rst
 ```
 
 ```{toctree}
-:caption: Extensions 
+:caption: Extensions
 :maxdepth: 1
 
 extensions