Move rustdoc to 'How to build the compiler'

The old layout was less about rustdoc and more about the compiler:
"Here is an example of a project using rustc_interface".
This gives focus to rustdoc on something you might want to work on for
its own sake. It also makes the page much easier to find for newcomers.

Author: jyn514

src/SUMMARY.md

@@ -14,6 +14,7 @@
         - [Bootstrapping](./building/bootstrapping.md)
         - [Distribution artifacts](./building/build-install-distribution-artifacts.md)
         - [Documenting Compiler](./building/compiler-documenting.md)
+        - [Rustdoc](./rustdoc.md)
         - [ctags](./building/ctags.md)
     - [The compiler testing framework](./tests/intro.md)
         - [Running tests](./tests/running.md)
@@ -54,7 +55,6 @@
 - [Part 3: Source Code Representations](./part-3-intro.md)
     - [Command-line arguments](./cli.md)
     - [The Rustc Driver and Interface](./rustc-driver.md)
-        - [Rustdoc](./rustdoc.md)
         - [Ex: Type checking through `rustc_interface`](./rustc-driver-interacting-with-the-ast.md)
         - [Ex: Getting diagnostics through `rustc_interface`](./rustc-driver-getting-diagnostics.md)
     - [Syntax and the AST](./syntax-intro.md)