[EPIC] Reformat primitives section

[kaelynj]

As we integrate the directed execution model (executor) into docs, we should reorder how the section on primitives is arranged. The end goal is to have a page ordering like:

└── Execute on QPUs V
    ├── Introduction V
    │   ├── What is a primitive?
    │   ├── Why did Qiskit introduce primitives?
    │   ├── Brief description of each primitive & when to use which
    │   └── Configure error suppression and mitigation
    ├── Sampler V
    │   ├── intro/hello world
    │   ├── inputs and outputs
    │   ├── options
    │   ├── e2e examples
    │   ├── BackendSampler → will move to the integrations sections 
    │   └── REST API
    ├── Estimator V
    │   ├── intro/hello world
    │   ├── inputs and outputs
    │   ├── options
    │   ├── e2e examples
    │   ├── BackendSampler → will move to the integrations sections 
    │   └── REST API
    └── Executor V
        ├── intro/hello world
        ├── inputs and outputs
        ├── options
        ├── e2e examples
        ├── BackendSampler → will move to the integrations sections 
        └── REST API

To keep our sanity and not blow this work up into a single PR, each step is laid out in sub-issues below. The first one (#4614 ) should be completed before starting the rest.

Once that's done, and after the TOC reordering is finalized, we should also move all sections in Introduction to primitives to Qiskit section (save for Primitive details).