Document how to produce a C4 diagram

docs/nodes/c4.md

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+---
+id: c4
+title: C4
+---
+
+## C4 Diagrams
+
+[C4](https://c4model.com/) is a standardized model to visualize software architecture.
+You can generate C4 diagrams by using the node and edge classes from the `diagrams.c4` package:
+
+```python
+from diagrams import Diagram
+from diagrams.c4 import Person, Container, Database, System, SystemBoundary, Relationship
+
+graph_attr = {
+    "splines": "spline",
+}
+
+with Diagram("Container diagram for Internet Banking System", direction="TB", graph_attr=graph_attr):
+    customer = Person(
+        name="Personal Banking Customer", description="A customer of the bank, with personal bank accounts."
+    )
+
+    with SystemBoundary("Internet Banking System"):
+        webapp = Container(
+            name="Web Application",
+            technology="Java and Spring MVC",
+            description="Delivers the static content and the Internet banking single page application.",
+        )
+
+        spa = Container(
+            name="Single-Page Application",
+            technology="Javascript and Angular",
+            description="Provides all of the Internet banking functionality to customers via their web browser.",
+        )
+
+        mobileapp = Container(
+            name="Mobile App",
+            technology="Xamarin",
+            description="Provides a limited subset of the Internet banking functionality to customers via their mobile device.",
+        )
+
+        api = Container(
+            name="API Application",
+            technology="Java and Spring MVC",
+            description="Provides Internet banking functionality via a JSON/HTTPS API.",
+        )
+
+        database = Database(
+            name="Database",
+            technology="Oracle Database Schema",
+            description="Stores user registration information, hashed authentication credentials, access logs, etc.",
+        )
+
+    email = System(name="E-mail System", description="The internal Microsoft Exchange e-mail system.", external=True)
+
+    mainframe = System(
+        name="Mainframe Banking System",
+        description="Stores all of the core banking information about customers, accounts, transactions, etc.",
+        external=True,
+    )
+
+    customer >> Relationship("Visits bigbank.com/ib using [HTTPS]") >> webapp
+    customer >> Relationship("Views account balances, and makes payments using") >> [spa, mobileapp]
+    webapp >> Relationship("Delivers to the customer's web browser") >> spa
+    spa >> Relationship("Make API calls to [JSON/HTTPS]") >> api
+    mobileapp >> Relationship("Make API calls to [JSON/HTTPS]") >> api
+
+    api >> Relationship("reads from and writes to") >> database
+    api >> Relationship("Sends email using [SMTP]") >> email
+    api >> Relationship("Makes API calls to [XML/HTTPS]") >> mainframe
+    customer << Relationship("Sends e-mails to") << email
+```
+
+It will produce the following diagram:
+
+![c4](/img/c4.png)