docs: document the workchain context nesting

docs/source/topics/workflows/include/snippets/workchains/run_workchain_submit_parallel_nested.py

@@ -0,0 +1,23 @@
+# -*- coding: utf-8 -*-
+from aiida.engine import WorkChain
+
+
+class SomeWorkChain(WorkChain):
+
+    @classmethod
+    def define(cls, spec):
+        super().define(spec)
+        spec.outline(
+            cls.submit_workchains,
+            cls.inspect_workchains,
+        )
+
+    def submit_workchains(self):
+        for i in range(3):
+            future = self.submit(SomeWorkChain)
+            key = f'workchain.sub{i}'
+            self.to_context(**{key: future})
+
+    def inspect_workchains(self):
+        for i in range(3):
+            assert self.ctx.workchain[f'sub{i}'].is_finished_ok

docs/source/topics/workflows/usage.rst

@@ -417,6 +417,17 @@ The ``self.ctx.workchains`` now contains a list with the nodes of the completed
 Note that the use of ``append_`` is not just limited to the ``to_context`` method.
 You can also use it in exactly the same way with ``ToContext`` to append a process to a list in the context in multiple outline steps.
 
+Nested context keys
+^^^^^^^^^^^^^^^^^^^
+
+To simplify the organization of the context, the keys may contain dots ``.``, transparently creating namespaces in the process.
+As an example compare the following to the parallel submission example above:
+
+.. include:: include/snippets/workchains/run_workchain_submit_append.py
+    :code: python
+
+This allows to create intuitively grouped and easily accessible structures of child calculations or workchains.
+
 .. _topics:workflows:usage:workchains:reporting:
 
 Reporting