PY-33341, PY-56416, PY-28900: Render attributes and init params description in the class documentation

PY-33341: Now we render the “Attributes” section in the class documentation, it also allows to describe inherited attributes. (Previously we didn’t render it at all)
PY-56416: In the attribute documentation popup we are able to render attribute description from class docstring. (Previously we took documentation only explicitly from attribute one-line docstring)
PY-33341: We render the “Parameters” section in the class documentation for init parameters, described in the class docstring. (Previously user couldn't use class docstring to describe init parameters)
PY-28900: For the init parameter documentation we take the description from the class docstring if init doesn't have its own docstring. (Previously we took parameter description only from init docstring)

GitOrigin-RevId: d67bf49c72cf7a3634805a6e310c943f1ea848d1

python/testData/quickdoc/InstanceAttributeDescriptionFromClassDocstringRest.py

@@ -0,0 +1,7 @@
+class MyClass:
+    """
+    :ivar attr: attr description
+    """
+
+    def __init__(self):
+        self.attr<the_ref> = 0