Fix pbcgto dead link and remove pbc gto duplicate pages

Author: tberkel

source/user/pbc.rst

@@ -11,7 +11,7 @@ which has a directory structure that mirrors that of the molecular :mod:`pyscf`
 module, e.g. ``from pyscf.pbc import gto, scf``.
 Details of how to specify the system details, including the unit cell,
 basis sets, and pseudopotentials, are provided in
-:ref:`user_pbc_gto`.
+`Crystal structure <pbcgto.html>`_.
 
 All electronic structure methods can be applied with periodic
 boundary conditions at the Gamma point of the Brillouin zone, :math:`k=(0,0,0)`,

source/user/pbc/gto.rst

@@ -76,10 +76,11 @@ PySCF uses crystalline Gaussian-type orbitals as basis functions for periodic
 calculations.  The predefined basis sets and ECPs for molecular calculations
 can be used in periodic calculations as well. 
 
-As described more in [](link), many PBC calculations require the use of
-ECPs (or pseudopotentials, as they are more commonly called in periodic codes).
-In addition to molecular ECPs, PySCF includes GTH pseudopotentials,
-which have been parameterized for use with HF or different DFT functionals,
+As described more in [Density fitting for crystalline
+calculations](pbc/df), many PBC calculations require the use of ECPs (or
+pseudopotentials, as they are more commonly called in periodic codes).  In
+addition to molecular ECPs, PySCF includes GTH pseudopotentials, which have
+been parameterized for use with HF or different DFT functionals,
 ```python
 cell.pseudo = 'gth-hf'
 cell.pseudo = 'gth-lda' # an alias for 'gth-pade'