Build the rustdoc docs with rustdoc; sundown requires matching
codefences.

Previously

    ~~~rust
    ```

was a valid code-block, which makes it impossible to render the docs for
rustdoc itself (as it has to have nested blocks to demonstrate testable
code blocks). This modifies sundown that the opening and closing blocks
are required to match.

This then allows rustdoc.md to be rendered with rustdoc and we can
complete the circle of dogfooding.