Clarify Read the Docs versioning and retention of older versions

.readthedocs.yaml

@@ -1,5 +1,7 @@
 # Read the Docs configuration file
 # See https://docs.readthedocs.io/en/stable/config-file/v2.html for details
+# Documentation on Read the Docs is intended to be built from released
+# git tags to support versioned documentation.
 
 version: 2
 

docs/source/index.rst

@@ -117,3 +117,17 @@ Indices and tables
 
 * :ref:`genindex`
 * :ref:`search`
+
+Documentation versions
+----------------------
+
+Mypy documentation on Read the Docs is built from released git tags.
+This allows users to browse documentation corresponding to older
+released versions of mypy.
+
+Using git tags for documentation versions avoids relying on long-lived
+maintenance branches and makes it easier to remove stale branches while
+still preserving historical documentation.
+
+Older documentation versions are kept available to support users
+working with pinned or legacy mypy releases.