Clarify qiskit metapackage confusion

[javabster]

We keep getting older users of qiskit being confused about the fact that when the do pip install qiskit they dont get Aer and the application modules installed as well anymore. We've talked about this before in various release blogs but there are still people who haven't got the message. We also still get a lot of users complaining about the application modules who don't understand why they can't find the docs in IQP or why those modules aren't qiskit 1.0 compatible. This is a pervasive issue that requires multiple solutions:

1. Update intro to qiskit page to include the pip install code snippets for each package
2. create a dedicated migration guide that explains the old metapackage structure and the new situation (and what happened to the application modules)
3. add a section to FAQs called "what happened to the application modules?" or something like that

Existing resources to base the copy on:
TODO: add links

[javabster]

one example of negative user feedback on this issue:

You are making this too complicated for first time users to understand and write or copy the the code into a py.script
What is needed is an up to date pip command which gives a user all the latest packages and examples of code//
This code is always returning errors//
from qiskit_aer import Aer # Import Aer from qiskit_aer
from qiskit_ibm_runtime import QiskitRuntimeService
from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit, execute
Error// from qiskit import QuantumRegister, ClassicalRegister, QuantumCircuit, execute
ImportError: cannot import name 'execute' from 'qiskit

(although this person has also missed the message that execute was deprecated over a year ago)