Audit and make documentation for command EXISTS consistent

[arpitbbhayani]

The documentation of the command [EXISTS](https://dicedb.io/commands/exists/) present in this directory could have become stale. We need your help to ensure that the documentation is complete, correct, and consistent.  Completing this issue will give you much-needed exposure to how the command is implemented and the different parameters it accepts. Hence, this is a great way to build a good understanding of the project and its functionality. While still making a significant contribution. Here's what you need to do:

1. Go through the documentation of the command and run all the examples, making sure they all work as expected
2. If the same command is present in Redis, then the expected output of the DiceDB command is the same as the Redis output
3. If the command is not in Redis, then as per your judgment, raise an issue with this template or fix the documentation and raise a pull request in the docs repository.
4. The documentation should contain the following sections in the following order

• short and concise introduction paragraph about the command covering what it does
• Syntax
• Parameters: List all the parameters the command accepts, and ignore the section if there are no parameters
• Return values: List all possible return values and under what condition
• Behaviour: Describe the behavior of the command, what it does, how it does it, some internal specifics if any
• Errors: List all possible errors the command can throw and under what condition
• Examples: List all possible examples of the command and the expected output. Assume CLI implementation

Note: The title of the sections should be the exact strings mentioned above, like "Examples", "Return values", etc.
Note: The description of the command in Frontmatter of the command.md file is the first paragraph (introduction) of the command.

Go through the DiceDB/dice repository and explore the command implementation

• to understand all possible return values
• to understand all possible error values and conditions
• to understand different parameters the command accepts

Please use the documentation of the SET command as the reference point and structure the
documentation of this command in a very similar way, including the following points

• if you see any "Conclusion" section, remove it
• the headers (h1, h2, h3) should be appropriately used
• the CLI prompt should be 127.0.0.1:7379> and not anything else
• Use markdown tables for parameters and return values (ref SET command documentation)
• wrap any command or paramter in backticks (`) to highlight them
• the section header should be capitalize as used in the SET command documentation

If you find any inconsistencies, please fix the documentation in the DiceDB/docs repository and raise the PR.
The core idea of this exercise is to ensure that the documentation is consistent, correct, and complete.

Note: The PR for changes in documentation needs to be raised in the docs repository while issue or fix in core database implementation will be raised in the dice repository.

Make sure you comment on the other issues you created (if any) as a comment on this issue and also any PR (if any) that you created.
Thank you for picking this up and contributing to the DiceDB. It means a ton.

[FriendWhoCodes]

Hi @arpitbbhayani , I'd like to take this up, could you please assign me this? Thanks

[arpitbbhayani]

@FriendWhoCodes One slight change.

We have moved docs to the main Dice repository which can be found under the docs directory. The folder structure is the same and you can apply the same changes to the main repository.

Please comment on the same issue on the main Dice repository and raise the PR again. Once again, I'm sorry for the inconvenience.

We have taken this call to keep the doc and the code together allowing us to keep docs consistent with the code and everything to be part of one PR checklist.

Thanks for understanding.