Start adding C API documentation

docs/source/capi/index.rst

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+.. _capi:
+
+=====
+C API
+=====
+
+.. c:function:: int openmc_find(double* xyz, int rtype)
+
+   Return the ID of the cell/material containing a given point
+
+   :param xyz: Cartesian coordinates
+   :type xyz: double[3]
+   :param rtype: Which ID to return (1=cell, 2=material)
+   :type rtype: int
+   :rtype: int
+
+.. c:function:: void openmc_init(int intracomm)
+
+   Initialize OpenMC
+
+   :param intracomm: MPI intracommunicator
+   :type intracomm: int
+
+.. c:function:: void openmc_finalize()
+
+   Finalize a simulation
+
+.. c:function:: void openmc_reset()
+
+   Resets all tally scores
+
+.. c:function:: void openmc_run()
+
+   Run a simulation