Stop referencing V2 everywhere and move migration guide (#6737)

docs/incompatible_changes/v2_migration.rst

@@ -0,0 +1,99 @@
+.. V3 Migration Guide
+
+.. role:: python(code)
+    :language: python
+
+==========================
+Migrating cogs from Red V2
+==========================
+
+First, be sure to read :dpy_docs:`discord.py's migration guide <migrating_to_v1.html>`
+as that covers all of the changes to discord.py that will affect the migration process
+
+----------------
+Red as a package
+----------------
+
+V3 makes Red a package that is installed with :code:`pip`. Please
+keep this in mind when writing cogs as this affects how imports 
+should be done (for example, to import :code:`pagify` in V2, one
+would do :code:`from .utils.chat_formatting import pagify`; in
+V3, this becomes :code:`from redbot.core.utils.chat_formatting import pagify`)
+
+----------------
+Cogs as packages
+----------------
+
+V3 makes cogs into packages. See :doc:`/guide_cog_creation`
+for more on how to create packages for V3.
+
+------
+Config
+------
+
+Config is V3's replacement for :code:`dataIO`. Instead of fiddling with
+creating config directories and config files as was done in V2, V3's
+Config handles that whilst allowing for easy storage of settings on a
+per-server/member/user/role/channel or global basis. Be sure to check
+out :doc:`/framework_config` for the API docs for Config as well as a
+tutorial on using Config.
+
+*************
+V2 Data Usage
+*************
+
+There has been much conversation on how to bring V2 data into V3 and, officially, we recommend that cog developers
+make use of the public interface in Config (using the categories as described in these docs) rather than simply
+copying and pasting your V2 data into V3. Using Config as recommended will result in a much better experience for
+you in the long run and will simplify cog creation and maintenance.
+
+However.
+
+We realize that many of our cog creators have expressed disinterest in writing converters for V2 to V3 style data.
+As a result we have opened up config to take standard V2 data and allow cog developers to manipulate it in V3 in
+much the same way they would in V2. The following examples will demonstrate how to accomplish this.
+
+.. warning::
+
+    By following this method to use V2 data in V3 you may be at risk of data corruption if your cog is used on a bot
+    with multiple shards. USE AT YOUR OWN RISK.
+
+.. code-block:: python
+
+    from redbot.core import Config, commands
+
+
+    class ExampleCog(commands.Cog):
+        def __init__(self):
+            self.config = Config.get_conf(self, 1234567890)
+            self.config.init_custom("V2", 1)
+            self.data = {}
+
+        async def load_data(self):
+            self.data = await self.config.custom("V2", "V2").all()
+
+        async def save_data(self):
+            await self.config.custom("V2", "V2").set(self.data)
+
+
+    async def setup(bot):
+        cog = ExampleCog()
+        await cog.load_data()
+        await bot.add_cog(cog)
+
+----
+Bank
+----
+
+Bank in V3 has been split out from Economy. V3 introduces the ability
+to have a global bank as well as the ability to change the bank name
+and the name of the currency. Be sure to checkout :doc:`/framework_bank`
+for more on Bank
+
+-------
+Mod Log
+-------
+
+V3 introduces Mod Log as an API, thus allowing for cogs to add custom case
+types that will appear in a server's mod log channel. Be sure to checkout
+:doc:`/framework_modlog` for more on Mod Log`