Bump ffi from 1.9.14 to 1.13.1 in /docs (#110)

Bumps [ffi](https://github.com/ffi/ffi) from 1.9.14 to 1.13.1.
- [Release notes](https://github.com/ffi/ffi/releases)
- [Changelog](https://github.com/ffi/ffi/blob/master/CHANGELOG.md)
- [Commits](ffi/ffi@1.9.14...1.13.1)

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

docs/Gemfile.lock

@@ -2,7 +2,7 @@ GEM
   remote: https://rubygems.org/
   specs:
     colorator (1.1.0)
-    ffi (1.9.14)
+    ffi (1.13.1)
     forwardable-extended (2.6.0)
     jekyll (3.2.1)
       colorator (~> 1.0)