minor #3767 [Book] link to the bc promise in the stable API descripti…

…on (xabbuh)

This PR was merged into the 2.3 branch.

Discussion
----------

[Book] link to the bc promise in the stable API description

| Q             | A
| ------------- | ---
| Doc fix?      | yes
| New docs?     | no
| Applies to    | all
| Fixed tickets |

Commits
-------

43761b7 [Book] link to the bc promise in the stable API description

book/stable_api.rst

@@ -18,6 +18,10 @@ in the stable API is in order to fix a security issue.
 The stable API is based on a whitelist, tagged with `@api`. Therefore,
 everything not tagged explicitly is not part of the stable API.
 
+.. tip::
+
+    Read more about the stable API in :doc:`/contributing/code/bc`.
+
 .. tip::
 
     Any third party bundle should also publish its own stable API.