Mention the need for the graphviz library on transpile/custom-backend page (#1558)

Closes #1394

Author: abbycross

docs/transpile/custom-backend.ipynb

@@ -39,7 +39,11 @@
     "\n",
     "The `BackendV2` object is an abstract class used for all backend objects created by a provider (either within `qiskit.providers` or another library such as [`qiskit_ibm_runtime.IBMBackend`](../api/qiskit-ibm-runtime/qiskit_ibm_runtime.IBMBackend)).  As mentioned above, these objects contain several attributes, including a [`Target`](/api/qiskit/qiskit.transpiler.Target). The `Target` contains information that specifies the backend's attributes - such as the [`Coupling Map`](/api/qiskit/qiskit.transpiler.CouplingMap), list of [`Instructions`](/api/qiskit/qiskit.circuit.Instruction), and others - to the transpiler.  In addition to the `Target`, one can also define pulse-level details such as the [`DriveChannel`](/api/qiskit/qiskit.pulse.channels.DriveChannel) or [`ControlChannel`](/api/qiskit/qiskit.pulse.channels.ControlChannel).\n",
     "\n",
-    "The following example demonstrates this customization by creating a simulated multi-chip backend, where each chip possesses a heavy-hex connectivity.  The example specifies the backend's two-qubit gate set to be [`CZGates`](../api/qiskit/qiskit.circuit.library.CZGate) within each chip and [`CXGates`](../api/qiskit/qiskit.circuit.library.ECRGate) between chips.  First, create your own `BackendV2` and customize its `Target` with single and two-qubit gates according to the previously described constraints."
+    "The following example demonstrates this customization by creating a simulated multi-chip backend, where each chip possesses a heavy-hex connectivity.  The example specifies the backend's two-qubit gate set to be [`CZGates`](../api/qiskit/qiskit.circuit.library.CZGate) within each chip and [`CXGates`](../api/qiskit/qiskit.circuit.library.ECRGate) between chips.  First, create your own `BackendV2` and customize its `Target` with single and two-qubit gates according to the previously described constraints.\n",
+    "\n",
+    "<Admonition type=\"tip\" title=\"graphviz library\">\n",
+    "Plotting a coupling map requires the [`graphviz`](https://graphviz.org/) library to be installed.\n",
+    "</Admonition>"
    ]
   },
   {

docs/transpile/custom-transpiler-pass.ipynb

@@ -20,7 +20,7 @@
     "    Background: DAG representation\n",
     "  </summary>\n",
     "\n",
-    "Before building a pass, it is important to introduce the internal representation of quantum circuits in Qiskit, the [directed acyclic graph (DAG)](../api/qiskit/qiskit.dagcircuit.DAGCircuit) (see [this tutorial](https://qiskit.org/ecosystem/rustworkx/tutorial/dags.html) for an overview).  To follow these steps, install the `graphivz` library for the DAG plotting functions.  Use a Python package manager (such as `pip` or `conda`) to install `pydot` and your system's native package manager (for example, `apt`, `brew`, `yum`, or `dnf`) for `graphivz`.\n",
+    "Before building a pass, it is important to introduce the internal representation of quantum circuits in Qiskit, the [directed acyclic graph (DAG)](../api/qiskit/qiskit.dagcircuit.DAGCircuit) (see [this tutorial](https://qiskit.org/ecosystem/rustworkx/tutorial/dags.html) for an overview).  To follow these steps, install the [`graphviz` library](https://graphviz.org/download/) for the DAG plotting functions.\n",
     "\n",
     "In Qiskit, within the transpilation stages, circuits are represented using a DAG.  In general, a DAG is composed of *vertices* (also known as \"nodes\") and directed *edges* that connect pairs of vertices in a particular orientation.  This representation is stored using `qiskit.dagcircuit.DAGCircuit` objects that are composed of invididual `DagNode` objects.  The advantage of this representation over a pure list of gates (that is, a *netlist*) is that the flow of information between operations is explicit, making it easier to make transformation decisions.\n",
     "\n",

docs/transpile/index.mdx

@@ -23,7 +23,7 @@ A central component of the Qiskit SDK, the transpiler is designed for modularity
 
 Find more information about the pass manager stages in the [Transpiler stages](transpiler-stages) topic.
 
-### Instruction set architecture
+## Instruction set architecture
 In addition to reducing the depth and complexity of quantum circuits, the transpiler is designed to transform the instructions contained in a given `QuantumCircuit` to obey the Instruction Set Architecture (ISA) of a particular backend.  Circuits obeying the ISA consist only of instructions that are supported by the backend's [`Target`](../api/qiskit/qiskit.transpiler.Target), such as the hardware's available basis gates, measurements, resets, and control flow operations, and comply with the constraints specified by the connectivity of the hardware, that is, the target's [`CouplingMap`](../api/qiskit/qiskit.transpiler.CouplingMap).  When submitting a job to an IBM Quantum™ backend, the circuits must adhere to the backend's ISA.