DOC-5378 reactive JSON doc examples

[andy-stark-redis]

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DOC-5378

Reactive examples for the main JSON data type page and the Path sub-page.

All suggestions about style, etc, are welcome, but a couple of things to note are:

• I'm following Tisho's comment on the corresponding async JSON PR that it's OK to use toString() results in tests to simplify them, even though it's not good practice generally to rely on toString() being consistent.
• There are some places in the code where a System.out::println method reference works naturally. However, there are other places in the published example where the only code visible in a doOnNext() call is a lambda of the form r -> {System.out.println(r);}. This looks like it could be replaced by the same method reference. However, it actually happens because there are also asserts in a REMOVE_START/REMOVE_END block in the lambda after the println call. It works fine, but if you think it looks like bad style to have this mix of method references and equivalent lambdas, then I'll see what I can do to fix it.