docs(elsa): runs and instances

elsa/runs-and-instances.md

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+# Runs & instances
+
+Every dispatch of a published definition creates a **workflow instance** (a run).
+
+- `/flows/runs` lists the instances of the selected runtime.
+- `/flows/runs/:id` shows one instance: state, incidents, journal, executions
+  and the call stack.
+
+## Statuses
+
+- **Running** — the instance is still executing.
+- **Finished** — reached an end activity without incidents.
+- **Faulted** — one or more activities raised an incident.
+
+Open an incident to see the activity, the message and the stack trace captured
+by the engine.
+
+<div class="docs-note">
+Runs are stored by the engine per runtime. Switch the runtime in the page header
+to inspect the instances of a self-hosted engine.
+</div>