Added Core Concepts article to docs

Author: GlebChili

docfx_project/articles/overview/core_concepts/core_concepts.md

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+---
+uid: core_concepts_overview
+title: "Core Concepts"
+---
+
+# Core Concepts
+
+Gmod.NET modules are essentially .NET (currently version 6.0) class libraries which reference [GmodNET.API nuget package](https://www.nuget.org/packages/GmodNET.API/) as their dependency.
+Each module should contain one or more implementation of [GmodNET.API.IModule](xref:GmodNET.API.IModule) interface.
+Gmod.NET runtime will instantiate each class derived from `IModule` by calling its parameterless constructor.
+
+## Module lifecycle
+
+At some moment Gmod.NET runtime will call each module's [Load](xref:GmodNET.API.IModule#GmodNET_API_IModule_Load_GmodNET_API_ILua_System_Boolean_GmodNET_API_ModuleAssemblyLoadContext_) method.
+```csharp
+public void Load(ILua lua, bool is_serverside, ModuleAssemblyLoadContext assembly_context)
+```
+This method can be thought as an entry point of the module where all initial setup is performed.
+The `lua` parameter will receive an instance of [GmodNET.API.ILua](xref:GmodNET.API.ILua) interface which should be used to communicate with Garry's Mod.
+The `is_serverside` parameter can be used to easily check whether module is running in server or client environment.
+The `assembly_context` parameter receives an instance of [GmodNET.API.ModuleAssemblyLoadContext](xref:GmodNET.API.ModuleAssemblyLoadContext) in which module was loaded,
+which can be used for some advanced scenarios or debugging.
+
+All modules also must implement [Unload](xref:GmodNET.API.IModule#GmodNET_API_IModule_Unload_GmodNET_API_ILua_) method.
+```csharp
+public void Unload(ILua lua);
+```
+This method is called by Gmod.NET runtime whenever module is unloaded manually or game/server session is over.
+The `lua` parameter receives an instance of [GmodNET.API.ILua](xref:GmodNET.API.ILua) interface which can be used to safely perform clenup tasks,
+like unregistering callbacks, removing data from global tables, etc.
+
+In order to load Gmod.NET module one should load Gmod.NET runtime via Lua using `require` function:
+```lua
+require("dotnet")
+```
+Then module can be loaded via Lua [dotnet.load](xref:lua_api_dotnet#loadstring) function.
+Module also can be unloaded with Lua [dotnet.unload](xref:lua_api_dotnet#unloadstring) function.
+Generally, modules can be loaded and unloaded at any moment, thus enabling hot-reloading and hot-switching for faster development.
+
+## Module file-structure and dependencies
+
+Modules can bring any .NET or native dependencies (usually acquired as NuGet packages).
+
+Modules' binaries and their placement must satisfy the following convention: module's main dll, its `%modules_name%.deps.json`,
+and all dependencies must be placed in `Garrys_mod_root_folder/garrysmod/lua/bin/Modules/%exact_name_of_your_module_without_dll%/` folder.
+For example, if your module is called `TestNETModule.dll` and it depends on `SomeLibrary.dll`,
+then `TestNETModules.dll`, `TestNETModule.deps.json`, `GmodNET.API.dll` and `SomeLibrary.dll` must be in `garrysmod/lua/bin/Modules/TestNETModule/` folder.
+Missing dependencies, `deps.json` files, or wrong folder naming may lead to inability of Gmod.NET runtime to load module.
+
+Gmod.NET runtime will locate and load module's managed and native dependencies into isolated `AssemblyLoadContxt` by reading `*.deps.json` file and using [standard .NET dependency resolution process](https://github.com/dotnet/runtime/blob/main/docs/design/features/host-probing.md).
+
+If Development environment is enabled, Gmod.NET runtime can load modules from any location, not only `garrysmod/lua/bin/Modules` folder.
+For more info see [Development environment feature](xref:runtime_features_development_environment).
+
+## Module permissions and safety
+
+Under the hood Gmod.NET uses standard Garry's Mod C++ API.
+It means that all Gmod.NET modules have the same permissions and privileges as host Garry's Mod process, including filesystem access, networking, etc.
+Thus, ***never run untrusted Gmod.NET modules or start Garry's Mod as an administrator or root***.

docfx_project/articles/overview/overview.md

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+---
+uid: overview
+title: Overview
+---
+
+# Overview
+
+This section contains brief overviews of concepts in Gmod.NET.

docfx_project/articles/overview/toc.yml

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+- name: "Core Concepts"
+  topicUid: core_concepts_overview

docfx_project/articles/toc.yml

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   href: tutorials/toc.yml
   topicHref: tutorials/overview.md
 
+- name: Overview
+  href: overview/toc.yml
+  topicHref: overview/overview.md
+
 - name: Runtime Features
   href: runtime_features/toc.yml
-  topicHref: runtime_features/overview.md
+  topicHref: runtime_features/overview.md