Add link to Utility Types page

Closes microsoft/TypeScript-Handbook#994
Closes microsoft/TypeScript-Handbook#1037

Author: wahahayay

src/_data/doctoc.yml

@@ -59,6 +59,7 @@
   - mixins
   - triple-slash-directives
   - type-checking-javascript-files
+  - utility-types
 - title: Declaration Files
   id: declaration-files
   directory: handbook/declaration-files
@@ -80,4 +81,4 @@
   - project-references
   - compiler-options-in-msbuild
   - integrating-with-build-tools
-  - nightly-builds
+  - nightly-builds