Move the bootstrapping section later in the guide (#975)

Now that `x.py build` works in most cases, I don't think we need to
teach people all the intricacies of bootstrap right away. This moves it
to the 'overview of the compiler' section instead.

Author: jyn514

src/SUMMARY.md

@@ -11,7 +11,6 @@
 - [How to Build and Run the Compiler](./building/how-to-build-and-run.md)
     - [Prerequisites](./building/prerequisites.md)
     - [Suggested Workflows](./building/suggested.md)
-    - [Bootstrapping](./building/bootstrapping.md)
     - [Distribution artifacts](./building/build-install-distribution-artifacts.md)
     - [Documenting Compiler](./building/compiler-documenting.md)
     - [Rustdoc](./rustdoc.md)
@@ -53,6 +52,7 @@
 - [Prologue](./part-2-intro.md)
 - [Overview of the Compiler](./overview.md)
 - [The compiler source code](./compiler-src.md)
+- [Bootstrapping](./building/bootstrapping.md)
 - [Queries: demand-driven compilation](./query.md)
     - [The Query Evaluation Model in Detail](./queries/query-evaluation-model-in-detail.md)
     - [Incremental compilation](./queries/incremental-compilation.md)