docs: Improved DI documentation (#2407)

docs/guides/dependency_injection/basics.md

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+---
+uid: Guides.DI.Intro
+title: Introduction
+---
+
+# Dependency Injection
+
+Dependency injection is a feature not required in Discord.Net, but makes it a lot easier to use.
+It can be combined with a large number of other libraries, and gives you better control over your application.
+
+> Further into the documentation, Dependency Injection will be referred to as 'DI'.
+
+## Installation
+
+DI is not native to .NET. You need to install the extension packages to your project in order to use it:
+
+- [Meta](https://www.nuget.org/packages/Microsoft.Extensions.DependencyInjection/).
+- [Abstractions](https://www.nuget.org/packages/Microsoft.Extensions.DependencyInjection.Abstractions/).
+
+> [!WARNING]
+> Downloading the abstractions package alone will not give you access to required classes to use DI properly.
+> Please install both packages, or choose to only install the meta package to implicitly install both.
+
+### Visual Package Manager:
+
+[Installing](images/manager.png)
+
+### Command Line:
+
+`PM> Install-Package Microsoft.Extensions.DependencyInjection`.
+
+> [!TIP]
+> ASP.NET already comes packed with all the necessary assemblies in its framework.
+> You do not require to install any additional NuGet packages to make full use of all features of DI in ASP.NET projects.
+
+## Getting started
+
+First of all, you will need to create an application based around dependency injection,
+which in order will be able to access and inject them across the project.
+
+[!code-csharp[Building the Program](samples/program.cs)]
+
+In order to freely pass around your dependencies in different classes,
+you will need to register them to a new `ServiceCollection` and build them into an `IServiceProvider` as seen above.
+The IServiceProvider then needs to be accessible by the startup file, so you can access your provider and manage them.
+
+[!code-csharp[Building the Collection](samples/collection.cs)]
+
+As shown above, an instance of `DiscordSocketConfig` is created, and added **before** the client itself is.
+Because the collection will prefer to create the highest populated constructor available with the services already present,
+it will prefer the constructor with the configuration, because you already added it.
+
+## Using your dependencies
+
+After building your provider in the Program class constructor, the provider is now available inside the instance you're actively using.
+Through the provider, we can ask for the DiscordSocketClient we registered earlier.
+
+[!code-csharp[Applying DI in RunAsync](samples/runasync.cs)]
+
+> [!WARNING]
+> Service constructors are not activated until the service is **first requested**.
+> An 'endpoint' service will have to be requested from the provider before it is activated.
+> If a service is requested with dependencies, its dependencies (if not already active) will be activated before the service itself is.
+
+## Injecting dependencies
+
+You can not only directly access the provider from a field or property, but you can also pass around instances to classes registered in the provider.
+There are multiple ways to do this. Please refer to the
+[Injection Documentation](Guides.DI.Injection) for further information.