Add documentation for GPCConcrete01 uniaxial material

[alpraldmr]

Documentation for the GPCConcrete01 uniaxial material (geopolymer
concrete with optional confinement) proposed in OpenSees/OpenSees#1829,
as requested by @fmckenna.

• Adds source/user/manual/material/uniaxialMaterials/GPCConcrete01.rst
• Adds the page to the uniaxialMaterial command index

Notes:

• This documents the command as proposed in Add GPCConcreteUnconfined01 and GPCConcreteConfined01 uniaxial materials (geopolymer concrete) [v2] OpenSees#1829, which
  has not been merged yet. This PR should be merged together with or
  after that one.
• The command syntax may still change while the code PR is under review;
  I will update this page to match.
• Only a Tcl example is included.
• Only files under source/ are changed (doc/ is untouched).