Add modularized functions to the DDD user guide

[purva-thakre]

Follow up from #2562

After newer modularized functions are added in ddd/, we want to make sure the newer functions are demoed in the user guide for DDD.

[Shivansh20128]

Hi @purva-thakre! Can you assign this to me?

[Shivansh20128]

Hi! @purva-thakre I have a doubt regarding these new functions.
So, the function generate_circuits_with_ddd generates a list of circuits and it needs a base circuit, a rule, and the number of trials as parameters. And we can provide the rule as per our choice as xx or yy etc. That will generate a new sample circuit with the DDD sequence plugged into the original circuit. Until here, it is fine.
But now to execute this circuit, we will call execute_with_ddd, which will take our new sample circuit, an executor, and a rule as parameters. This makes me question how the 'rule' is functioning here because we already have a DDD sequence plugged into the circuit. Are we plugging in a DDD sequence again while executing?
Since these parameters are not optional, we need to pass something as a rule for both these functions.

[purva-thakre]

Pinging @bdg221 to answer the above as you worked on the modularized functions for DDD.

[bdg221]

Hey @Shivansh20128, the point of the modularized functions is so that users don't have to call execute_with_ddd which is more of a black box. Instead, we have broken the pieces out (generate_circuits_with_ddd and combine_results) and along with calling the executor, the users can perform the function of execute_with_ddd manually.

If you check the ZNE example, you can see how the circuits are generated and execute(circuit) is called.

[Shivansh20128]

Got it. I was unaware of the function execute(circuit). Thank you @bdg221.