Automate iOS integration steps 5 and 6

[bdeweygit]

1. Added Assets/Editor/iOSPBXProjectModifier.cs build post-processor script to automate steps 5 and 6 from the iOS integration docs. Before, these steps would need to be done manually each time we generated a replacement Unity build. Now, they are done automatically.

   • Uses the UnityEditor.Callbacks.PostProcessBuildAttribute. Alternatively we could use the UnityEditor.Build.IPostprocessBuildWithReport interface but documentation is not clear about which is preferred and there are some known bugs with the interface.
   • Named in the style of the existing Assets/Editor/AndroidGradleProjectModifier.cs script.
   • Derived from a similar script in another project here.

2. Removed steps 5 and 6 from the documentation.