docs: use PRPL pattern doc link (#10004)

Hey, just noticed the term PRPL pattern in "SEO with Gatsby", turns out we actually have another doc page about that.
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docs/docs/seo.md

@@ -12,7 +12,7 @@ Because Gatsby pages are server-rendered, all the page content is available to G
 
 ### Speed boost
 
-Gatsby's many built-in performance optimizations, such as rendering to static files, progressive image loading, and the PRPL pattern—all help your site be lightning-fast by default.
+Gatsby's many built-in performance optimizations, such as rendering to static files, progressive image loading, and the [PRPL pattern](/docs/prpl-pattern/)—all help your site be lightning-fast by default.
 
 Starting in January 2018, Google [rewards faster sites with a bump in search rankings](https://searchengineland.com/google-speed-update-page-speed-will-become-ranking-factor-mobile-search-289904).