Move "Getting Started" to the first page

This page is better suited for people landing immediately from
CONTRIBUTING.md, or who haven't heard of the guide before. "About This
Guide" is still useful, but doesn't need to be the very first page.

Author: jyn514

src/SUMMARY.md

@@ -1,8 +1,7 @@
 # Summary
 
-[About this guide](./about-this-guide.md)
-
 [Getting Started](./getting-started.md)
+[About this guide](./about-this-guide.md)
 
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