[Docs] Add bank docs (#900)

* Add bank docs

* [Docs] fix bank docstrings for formatting

* [Docs] add bank to toctree

* rename and make bank rst use members

* Formatting changes

* Add account to dunder all

docs/framework_bank.rst

@@ -0,0 +1,42 @@
+.. V3 Bank
+
+.. role:: python(code)
+    :language: python
+
+====
+Bank
+====
+
+Bank has now been separated from Economy for V3. New to bank is support for
+having a global bank.
+
+***********
+Basic Usage
+***********
+
+.. code-block:: python
+
+    from core import bank
+
+    class MyCog:
+        @commands.command()
+        async def balance(self, ctx, user: discord.Member=None):
+            if user is None:
+                user = ctx.author
+            bal = bank.get_balance(user)
+            currency = bank.get_currency_name(ctx.guild)
+            await ctx.send(
+                "{}'s balance is {} {}".format(
+                    user.display_name, bal, currency
+                )
+            )
+
+*************
+API Reference
+*************
+
+Bank
+======
+
+.. automodule:: core.bank
+    :members: