Ship the three C4 examples and model:scaffold --example

- examples/: c4-standard-web-app, c4-e-shop (was online-shop),
  c4-event-based-integration; the same models the extension offers
  under Archify | New Model…
- model-scaffold --example <name> copies one and renames id, name,
  description and the overview title; never overwrites
- tests cover every example and the copy; docs updated

examples/c4-e-shop/0100-context/online-shop.archify

@@ -0,0 +1,26 @@
+schema_version: 1
+diagram_type: architecture
+meta:
+  title: System context
+  output: context.html
+components:
+  - ref: customer
+    pos: [40, 60]
+    size: [130, 60]
+  - ref: support
+    pos: [40, 260]
+    size: [130, 60]
+  - ref: shop
+    pos: [280, 160]
+    size: [150, 60]
+  - ref: psp
+    pos: [520, 60]
+    size: [150, 60]
+  - ref: email
+    pos: [520, 260]
+    size: [150, 60]
+connections:
+  - { from: customer, to: shop, label: buys, variant: emphasis }
+  - { from: support, to: shop, label: manages orders }
+  - { from: shop, to: psp, label: charges, variant: security }
+  - { from: shop, to: email, label: sends mail }