Incorporate reply docs in command markdown files

[zuiderkwast]

The replies of each command is documented in the files resp2_replies.json and resp3_replies.json. This is messy and it's easy for contributors to miss them.

This structure is from some incomplete(?) refactoring in the Redis times. These files just contain markdown text wrapped in JSON. It doesn't add any value to have them separated out, but just extra indirection and complexity.

We can move this text into each command's markdown file under a ## Reply heading. This can be done semi-automatically with a manual review. I suggest we place it after the command description and parameters but before Notes and Examples. (This order is commonly used in man pages.)

Additional idea: When RESP2 and RESP3 is identical, we don't need to list them separately. For the minor differences between RESP2 and RESP3, for example nil in RESP2 and null in RESP3, we can converge on null so we don't need separate text just because of a small difference like this. This terminology can also be updated in the topics/protocol.md page.