Bump nokogiri from 1.13.4 to 1.13.6 in /docs (#554)

Author: dependabot[bot]

docs/Gemfile.lock

@@ -225,11 +225,11 @@ GEM
       jekyll-seo-tag (~> 2.1)
     minitest (5.14.4)
     multipart-post (2.1.1)
-    nokogiri (1.13.4-arm64-darwin)
+    nokogiri (1.13.6-arm64-darwin)
       racc (~> 1.4)
-    nokogiri (1.13.4-x86_64-darwin)
+    nokogiri (1.13.6-x86_64-darwin)
       racc (~> 1.4)
-    nokogiri (1.13.4-x86_64-linux)
+    nokogiri (1.13.6-x86_64-linux)
       racc (~> 1.4)
     octokit (4.21.0)
       faraday (>= 0.9)