Add Migration Guide for Opflow (Qiskit/qiskit#9549)

* Add draft

* Remove qiskit

* Apply review

* Apply comments boxnote

* Small edit

* Apply review

* Update up to primitive ops

* Dump changes from sphinx repo

* Add twisties

* Update information, add examples

* Formatting changes

* Simplify intro

* Add links

* Add sampler example

* Update docs/migration_guides/opflow_migration.rst

Co-authored-by: Steve Wood <40241007+woodsp-ibm@users.noreply.github.com>

* Update docs/migration_guides/opflow_migration.rst

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Update docs/migration_guides/opflow_migration.rst

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Update docs/migration_guides/opflow_migration.rst

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Update docs/migration_guides/opflow_migration.rst

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Apply suggestions from code review

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Update CircuitOp

Co-authored-by: Julien Gacon <gaconju@gmail.com>

* Applied suggestions locally

* Change format, links

* Add grouping, algorithms link

* Fix indentation

* Final review, add links

* Add gradients

* Apply heading suggestions

Co-authored-by: Junye Huang <h.jun.ye@gmail.com>

* Add tags

* Apply suggestions code review

* Correct code examples, add outputs

* Run code examples by doctest

* Remove underscores

* Apply suggestions second round of review

Co-authored-by: Junye Huang <h.jun.ye@gmail.com>

* Add globals link

* Change links to refs

* Add dropdowns

* Change doctest syntax, apply reviews

* Try CI config

* Add internal links

* Fix lint, add doctest import

* Fix cyclic import

* Fix cyclic import

* Apply suggestions from Declan's code review

Co-authored-by: Declan Millar <declan.millar@ibm.com>

* Update qiskit/quantum_info/__init__.py

* Fix import

* Copy docs config from Qiskit/qiskit#9716

* Make black

* Remove duplicate

* Reorder

---------

Co-authored-by: Steve Wood <40241007+woodsp-ibm@users.noreply.github.com>
Co-authored-by: Julien Gacon <gaconju@gmail.com>
Co-authored-by: Junye Huang <h.jun.ye@gmail.com>
Co-authored-by: Declan Millar <declan.millar@ibm.com>