DOC-5375 reactive hash examples

[andy-stark-redis]

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DOC-5375

Reactive examples for the hash doc page. Also fixed what I think is an incorrectly-placed REMOVE_START in the existing strings example.

Note: this is my first reactive example, so I'm not sure of the best style, etc. In particular:

• Does it look OK to have the block() calls visible in some of the examples? We could hide them, but I'm wondering if it emphasises that you have to wait for a command like hset to complete before you can hget the values it has set.
• I've used flatMap only when there is a strict sequence of operations (like with the chain of dependent hincr calls in the last example) rather than use it to recreate the exact sequence used in the equivalent Jedis examples.

Any other suggestions or corrections would be very welcome :-)