build(deps): bump mini_racer from 0.2.9 to 0.2.15 in /docs/v3

Bumps [mini_racer](https://github.com/discourse/mini_racer) from 0.2.9 to 0.2.15.
- [Release notes](https://github.com/discourse/mini_racer/releases)
- [Changelog](https://github.com/rubyjs/mini_racer/blob/master/CHANGELOG)
- [Commits](rubyjs/mini_racer@v0.2.9...v0.2.15)

Signed-off-by: dependabot[bot] <support@github.com>

Author: dependabot[bot]

docs/v3/Gemfile.lock

@@ -87,8 +87,8 @@ GEM
       middleman-core (>= 3.2)
       rouge (~> 3.2)
     mini_portile2 (2.4.0)
-    mini_racer (0.2.9)
-      libv8 (>= 6.9.411)
+    mini_racer (0.2.15)
+      libv8 (> 7.3)
     minitest (5.14.0)
     nokogiri (1.10.9)
       mini_portile2 (~> 2.4.0)