feat: initialize Geekeey Arcade .NET SDK repository

Single-commit squash of the repository history containing the Arcade
.NET MSBuild SDK packages (base, Razor, Web, Test), their analyzer and
editorconfig configuration, test suites, and CI workflows.

src/sdk/Sdk/Geekeey.Arcade.NET.Sdk.Web/PACKAGE.md

@@ -0,0 +1,29 @@
+# Geekeey.Arcade.NET.Sdk.Web
+
+Opinionated MSBuild SDK for ASP.NET Core web projects with standardized build, analyzer, and editor configuration defaults.
+
+## Getting started
+
+```xml
+<Project Sdk="Geekeey.Arcade.NET.Sdk.Web/1.0.0">
+	<PropertyGroup>
+		<TargetFramework>net8.0</TargetFramework>
+	</PropertyGroup>
+</Project>
+```
+
+## What it provides
+
+- Everything from the base `Geekeey.Arcade.NET.Sdk` (language, analyzer, editor, and packaging defaults), layered on top of the `Microsoft.NET.Sdk.Web` SDK.
+- Opinionated `.editorconfig` settings for web projects, applied automatically via `EditorConfigFiles` (including file-based programs when `FileBasedProgram=true`).
+
+## Opt-outs
+
+| Property | Effect |
+| --- | --- |
+| `EnableDefaultJsonSerializationOptions=false` | Skips the `System.Text.Json` nullable annotation defaults. |
+| `IncludeDefaultBannedSymbols=false` | Skips the built-in `BannedSymbols.txt`. |
+| `BannedNewtonsoftJsonSymbols=false` | Skips the built-in `BannedSymbols.NewtonsoftJson.txt`. |
+| `SearchReadmeFileAbove=true` | Also searches parent directories for a `PACKAGE.md` to pack. |
+
+See the [repository](https://code.geekeey.de/geekeey/arcade) for the full configuration.