Add "no-issue-needed" rule directly in CONTRIBUTING.rst (#23802)

The rule was not really explained directly where you'd expect it,
it was hidden deeply in "triage" process where many contributors
would not even get to.

This PR adds appropriate explanation and also explains that
discussions is the preferred way to discuss things in Airflow
rather than issues.

CONTRIBUTING.rst

@@ -66,6 +66,18 @@ implement it.
 Issue reporting and resolution process
 --------------------------------------
 
+An unusual element of the Apache Airflow project is that you can open a PR to
+fix an issue or make an enhancement, without needing to open an issue first.
+This is intended to make it as easy as possible to contribute to the project.
+
+If you however feel the need to open an issue (usually a bug or feature request)
+consider starting with a `GitHub Discussion <https://github.com/apache/airflow/discussions>`_ instead.
+In the vast majority of cases discussions are better than issues - you should only open
+issues if you are sure you found a bug and have a reproducible case,
+or when you want to raise a feature request that will not require a lot of discussion.
+If you have a very important topic to discuss, start a discussion on the
+`Devlist <https://lists.apache.org/list.html?dev@airflow.apache.org>`_ instead.
+
 The Apache Airflow project uses a set of labels for tracking and triaging issues, as
 well as a set of priorities and milestones to track how and when the enhancements and bug
 fixes make it into an Airflow release. This is documented as part of