Rewrite all concepts documentation

hello RC documentation

docs/guides/concepts/entities.md

@@ -0,0 +1,71 @@
+---
+title: Entities
+---
+
+>[!NOTE]
+This article is written with the Socket variants of entities in mind,
+not the general interfaces or Rest/Rpc entities.
+
+Discord.Net provides a versatile entity system for navigating the
+Discord API.
+
+### Inheritance
+
+Due to the nature of the Discord API, some entities are designed with
+multiple variants, for example, `SocketUser` and `SocketGuildUser`.
+
+All models will contain the most detailed version of an entity
+possible, even if the type is less detailed. 
+
+For example, in the case of the `MessageReceived` event, a 
+`SocketMessage` is passed in with a channel property of type
+`SocketMessageChannel`. All messages come from channels capable of
+messaging, so this is the only variant of a channel that can cover
+every single case.
+
+But that doesn't mean a message _can't_ come from a
+`SocketTextChannel`, which is a message channel in a guild. To
+retrieve information about a guild from a message entity, you will
+need to cast its channel object to a `SocketTextChannel`.
+
+### Navigation
+
+All socket entities have navigation properties on them, which allow
+you to easily navigate to an entity's parent or children. As explained
+above, you will sometimes need to cast to a more detailed version of
+an entity to navigate to its parent.
+
+All socket entities have a `Discord` property, which will allow you
+to access the parent `DiscordSocketClient`. 
+
+### Accessing Entities
+
+The most basic forms of entities, `SocketGuild`, `SocketUser`, and
+`SocketChannel` can be pulled from the DiscordSocketClient's global
+cache, and can be retrieved using the respective `GetXXX` method on
+DiscordSocketClient.
+
+>[!TIP]
+It is **vital** that you use the proper IDs for an entity when using
+a GetXXX method. It is recommended that you enable Discord's
+_developer mode_ to allow easy access to entity IDs, found in
+Settings > Appearance > Advanced
+
+More detailed versions of entities can be pulled from the basic
+entities, e.g. `SocketGuild.GetUser`, which returns a 
+`SocketGuildUser`, or `SocketGuild.GetChannel`, which returns a 
+`SocketGuildChannel`. Again, you may need to cast these objects to get
+a variant of the type that you need.
+
+### Samples
+
+[!code-csharp[Entity Sample](samples/entities.cs)]
+
+### Tips
+
+Avoid using boxing-casts to coerce entities into a variant, use the
+`as` keyword, and a null-conditional operator.
+
+This allows you to write safer code, and avoid InvalidCastExceptions.
+
+For example, `(message.Author as SocketGuildUser)?.Nickname`.