[Refactoring, Documentation] Clarify the `entangle` function

[jpmoutinho]

The entangle function is an operation available only for the Pulser backend which helps generate GHZ states with a specific sequence of pulses. We should:

• Give it a less vague and more formal name
• Implement it in PyQTorch
• Document it properly with the math details on what it is doing

[RolandMacDoland]

@jpmoutinho Is this still relevant ? Since entangle doesn't exist anymore.

[jpmoutinho]

It does still exist in the code base, maybe we can keep this around for the time being