Add GitBook scaffolding.

Author: alexrp

.gitbook.yaml

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+root: doc
+structure:
+    readme: index.md
+    summary: toc.md

doc/index.md

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+# Home
+
+The **Zig SDK** is an
+[MSBuild SDK](https://docs.microsoft.com/en-us/visualstudio/msbuild/how-to-use-project-sdk)
+that augments the .NET SDK with the ability to build Zig, C, and C++ projects.
+
+With support for multiple programming languages, cross-compilation, NuGet
+packaging, and more, the **Zig SDK** makes it trivial to author native
+components as part of your .NET solution - without all the hassle that is
+usually part and parcel of building and packaging native code.

doc/toc.md

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+# Table of Contents
+
+* [Home](index.md)