Document business process events (#30)

Author: lorenyu

template-only-docs/business-process.md

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+# Business Process
+
+## Initialization
+
+```mermaid
+sequenceDiagram
+  participant PBPM as PassportBusinessProcessManager
+  participant BP as BusinessProcess.initialize
+
+  note over PBPM: create passport business process
+  PBPM ->> BP: new BusinessProcess(steps, transitions)
+  note over BP: @steps = steps
+  note over BP: @transitions = transitions
+  note over BP: hook up event listeners
+  note over BP: for event_name in get_event_names() subscribe(event_name, self.handle_event)
+
+  BP ->> PBPM: return
+```
+
+## Response to event
+
+```mermaid
+sequenceDiagram
+  actor U as User
+  participant BP as BusinessProcess.handle_event
+  participant V as VerifyIdentityStep.execute
+  participant R as ReviewPhotoStep.execute
+
+  note over U: user submits application
+  U ->> BP: publish("app_submitted", PassportCase.find(app.case_id))
+
+  note over BP: next_step = @transitions[kase.current_step][event_name]
+  note over BP: kase.current_step = next_step
+
+  BP ->> V: @steps[next_step].execute(kase)
+  note over V: create a task
+
+  note over U: admin verifies identity
+  U ->> BP: publish("identity_verified", kase)
+
+  note over BP: next_step = @transitions[kase.current_step][event_name]
+  note over BP: kase.current_step = next_step
+
+  BP ->> R: @steps[next_step].execute(kase)
+  note over R: create a task
+```