Gamma ray documentation introduction (#2858)

* Added an introduction to the gamma ray documentation.

* Fixed typo

docs/physics/tardisgamma/index.rst

@@ -1,6 +1,11 @@
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 TARDIS gamma :math:`\gamma`
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+This code module simulates the propagation of :math:`\gamma`-ray photons through the ejecta and can be used to study high energy phenomenon within the ejecta as well as the energy deposition of :math:`\gamma`-rays and positrons.
+
+Type Ia supernovae produce a large amount of energy from :math:`\gamma`-rays produced from the radioactive decay of :sup:`56` Ni and :sup:`56` Co. These photons often are not able to escape the ejecta until later times in the supernova when the density decreases. Once the :math:`\gamma`-rays are able to escape they can provide information about the mass and distribution of radioactive isotopes created in the supernova.
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+
 
 
 .. toctree::