Add link to the actor module documentation

To be able to use the `Actor` class and `run()` function there is a lot
to have in mind that is hard to explain (without a lot of duplication)
in each docstring. Because of this we just add an information box
suggesting users to read the module-level documentation.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/sdk/actor/_actor.py

@@ -15,10 +15,16 @@
 class Actor(BackgroundService, abc.ABC):
     """A primitive unit of computation that runs autonomously.
 
-    To implement an actor, subclasses must implement the `_run()` method, which should
-    run the actor's logic. The `_run()` method is called by the base class when the
-    actor is started, and is expected to run until the actor is stopped.
+    To implement an actor, subclasses must implement the
+    [`_run()`][frequenz.sdk.actor--the-_run-method] method, which should run the actor's
+    logic. The [`_run()`][frequenz.sdk.actor--the-_run-method] method is called by the
+    base class when the actor is started, and is expected to run until the actor is
+    stopped.
 
+    !!! info
+
+        Please read the [`actor` module documentation][frequenz.sdk.actor] for more
+        comprehensive guide on how to use and implement actors properly.
     """
 
     _restart_limit: int | None = None

src/frequenz/sdk/actor/_run_utils.py

@@ -15,6 +15,11 @@
 async def run(*actors: Actor) -> None:
     """Await the completion of all actors.
 
+    !!! info
+
+        Please read the [`actor` module documentation][frequenz.sdk.actor] for more
+        comprehensive guide on how to use and implement actors properly.
+
     Args:
         *actors: the actors to be awaited.
     """