dataclasses docs: add some link anchors

These would be useful to link to in the typing spec.

Doc/library/dataclasses.rst

@@ -556,6 +556,8 @@ See the section below on init-only variables for ways to pass
 parameters to :meth:`!__post_init__`.  Also see the warning about how
 :func:`replace` handles ``init=False`` fields.
 
+.. _dataclasses-class-variables:
+
 Class variables
 ---------------
 
@@ -567,6 +569,8 @@ from consideration as a field and is ignored by the dataclass
 mechanisms.  Such ``ClassVar`` pseudo-fields are not returned by the
 module-level :func:`fields` function.
 
+.. _dataclasses-init-only:
+
 Init-only variables
 -------------------
 
@@ -598,6 +602,8 @@ value is not provided when creating the class::
 In this case, :func:`fields` will return :class:`Field` objects for ``i`` and
 ``j``, but not for ``database``.
 
+.. _dataclasses-frozen:
+
 Frozen instances
 ----------------
 
@@ -611,6 +617,8 @@ There is a tiny performance penalty when using ``frozen=True``:
 :meth:`~object.__init__` cannot use simple assignment to initialize fields, and
 must use :meth:`!__setattr__`.
 
+.. _dataclasses-inheritance:
+
 Inheritance
 -----------