Update Documentation related to callback functions

[brock]

If I have time, I'll open an MR for this later. Logging here though...

The on_before_send function on the RaygunSender and the before_send_callback option in the config are not immediately clear when reading the docs. It was only after I read the code that I understood that these are related.

It would be nice to ideally change the online documentation to split up the "Config and data functions" into two separate sections:

1. Rename Config to "Passing a config object to the RaygunSender client"
   a. Lead with an example of passing in one large config object with all options included.
   b. Inside each option, outline the types of data you can pass, with examples
2. Create a section called "Calling functions on the instantiated RaygunSender client"
   a. Show examples like client.on_before_send(callback) with similar example in here.