add conversions documentation

docs/conversion.txt

+========================
+GW Parameter Conversions
+========================
+
+Sometimes it is useful to sample in some parameters and return others in the output.
+E.g., sampling in chirp mass and mass ratio can be much more efficient than sampling in component masses.
+
+We have many functions to do this.
+
+.. automodule:: bilby.gw.conversion
+    :members:
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