Make JSONHandler customization docs clearer

[vytas7]

As pointed out by @Stargateur in https://github.com/falconry/falcon/issues/1906#issuecomment-817374057, our JSONHandler customization docs could be made clearer by separately illustrating different (albeit closely related) concepts:

• Use a custom JSON library (such as the exemplified rapidjson). Customize parameters.
• Use the stdlib's json module, just provide custom serialization or deserialization parameters. Also link to the "Prettifying JSON Responses" recipe, which illustrates customization of dumps parameters.
• Add a sentence or two about replacing the default JSON handlers, not just toss in a code snippet as it is at the time of writing this. Also link to Replacing the Default Handlers from that explanation.

[dflss]

Hi, I can start working on this

[vytas7]

Awesome, go ahead @dflss ! I'll remove the needs contributor label.