Merge branch 'add_conversion_documentation' into 'master'

add conversions documentation

See merge request !308

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:
\ No newline at end of file

docs/index.txt

@@ -17,6 +17,7 @@ Welcome to bilby's documentation!
    bilby-output
    compact-binary-coalescence-parameter-estimation
    transient-gw-data
+   conversion
    writing-documentation
    hyperparameters