Add flake8 tool instructions to migration guide (#875)

Adds instructions on using the flake8 plugin to detect deprecated
imports.

---------

Co-authored-by: Eric Arellano <14852634+Eric-Arellano@users.noreply.github.com>

cspell/dictionaries/qiskit.txt

@@ -48,6 +48,7 @@ multiqubit
 openqasm
 parameterizing
 Paulihedral
+pipx
 polynomially
 pygments
 pyplot

docs/api/migration-guides/qiskit-1.0-features.mdx

@@ -8,6 +8,58 @@ description: Description of feature changes introduced in Qiskit 1.0 and how to
 This guide describes migration paths for the most important feature changes in Qiskit®  1.0, organized by module.
 Use the table of contents on the right side to navigate to the module you are interested in.
 
+## Qiskit 1.0 migration tool
+
+To ease the migration process, you can use the
+[`flake8-qiskit-migration`](https://github.com/qiskit-community/flake8-qiskit-migration)
+tool to detect removed import paths in your code and suggest alternatives.
+
+<Tabs>
+  <TabItem value="flake8-pipx" label="Run with pipx" default>
+      If you have [`pipx`](https://github.com/pypa/pipx) installed, simply run
+      the following command.
+      ```sh
+      pipx run flake8-qiskit-migration <path-to-source-directory> 
+      ```
+      This will install the package to a temporary virtual environment and run
+      it on your code.
+  </TabItem>
+  <TabItem value="flake8-venv" label="Run with venv" default>
+      If you don't want to use `pipx`, you can manually create a new
+      environment for the tool. This approach also lets you use
+      [`nbqa`](https://github.com/nbQA-dev/nbQA) to check code examples in
+      Jupyter notebooks. Delete the environment when you're finished.
+      ```sh
+      # Make new environment and install
+      python -m venv .flake8-qiskit-migration-venv
+      source .flake8-qiskit-migration-venv/bin/activate
+      pip install flake8-qiskit-migration
+
+      # Run plugin on Python code
+      flake8 --select QKT100 <path-to-source-directory>  # e.g. `src/`
+
+      # (Optional) run plugin on notebooks
+      pip install nbqa
+      nbqa flake8 ./**/*.ipynb --select QKT100
+
+      # Deactivate and delete environment
+      deactivate
+      rm -r .flake8-qiskit-migration-venv
+      ```
+  </TabItem>
+</Tabs>
+
+<Admonition type="note" title="Limitations">
+This tool only detects removed import paths. It does not detect the use of removed
+methods (such as `QuantumCircuit.qasm`) or arguments. It also can't keep track
+of assignments such as `qk = qiskit`, although it can handle aliases such as
+`import qiskit as qk`.
+</Admonition>
+
+For more information, see the [project's
+repository](https://github.com/qiskit-community/flake8-qiskit-migration).
+
+
 <span id="qiskit.global"></span>
 ## Global instances and functions