docs/reference: document compat guarantee, importing, shapes.

This commit also contains a related semantic change: it adds `Shape`
and `ShapeCastable` to the `__all__` list in `amaranth.hdl`. This is
consistent with the policy that is laid out in the new documentation,
which permits such additions without notice.

Co-authored-by: mcclure <mcclure@users.noreply.github.com>

tests/test_hdl_ast.py

@@ -167,8 +167,8 @@ class MockShapeCastable(ShapeCastable):
 class ShapeCastableTestCase(FHDLTestCase):
     def test_no_override(self):
         with self.assertRaisesRegex(TypeError,
-                r"^Class 'MockShapeCastableNoOverride' deriving from `ShapeCastable` must "
-                r"override the `as_shape` method$"):
+                r"^Class 'MockShapeCastableNoOverride' deriving from 'ShapeCastable' must "
+                r"override the 'as_shape' method$"):
             class MockShapeCastableNoOverride(ShapeCastable):
                 def __init__(self):
                     pass