docs: begin building the documentation style guide.

amaranth/hdl/_ast.py

@@ -875,7 +875,7 @@ class Value(metaclass=ABCMeta):
 
         The shape of the result is the same as the shape of :py:`self`, even for unsigned values.
 
-        .. important::
+        .. warning::
 
             In Python, :py:`~0` equals :py:`-1`. In Amaranth, :py:`~C(0)` equals :py:`C(1)`.
             This is the only case where an Amaranth operator deviates from the Python operator

amaranth/hdl/_mem.py

@@ -14,7 +14,7 @@ __all__ = ["FrozenMemory", "MemoryData", "Memory", "ReadPort", "WritePort", "Dum
 
 @final
 class FrozenMemory(Exception):
-    """This exception is raised when a memory array is being modified after elaboration."""
+    """Exception raised when a memory array is being modified after elaboration."""
 
 
 @final
@@ -194,7 +194,7 @@ class MemoryData:
         value that can be used to read and write the selected memory row in a simulation testbench,
         without having to create a memory port.
 
-        .. important::
+        .. tip::
 
             Even in a simulation, the value returned by this function cannot be used in a module;
             it can only be used with :py:`sim.get()` and :py:`sim.set()`.

amaranth/lib/wiring.py

@@ -689,7 +689,7 @@ class Signature(metaclass=SignatureMeta):
     :class:`Signature` can be used as a base class to define :ref:`customized <wiring-customizing>`
     signatures and interface objects.
 
-    .. important::
+    .. warning::
 
         :class:`Signature` objects are immutable. Classes inheriting from :class:`Signature` must
         ensure this remains the case when additional functionality is added.
@@ -1143,7 +1143,7 @@ class PureInterface:
     :meth:`Signature.create`, but it can also be used in any other context where an interface
     object needs to be created without the overhead of defining a class for it.
 
-    .. important::
+    .. tip::
 
         Any object can be an interface object; it only needs a :py:`signature` property containing
         a compliant signature. It is **not** necessary to use :class:`PureInterface` in order to
@@ -1678,7 +1678,7 @@ class Component(Elaboratable):
     def signature(self):
         """The signature of the component.
 
-        .. important::
+        .. warning::
 
             Do not override this property. Once a component is constructed, its :attr:`signature`
             property must always return the same :class:`Signature` instance. The constructor