Bump sprockets from 3.7.0 to 4.0.0 in /docs

Bumps [sprockets](https://github.com/rails/sprockets) from 3.7.0 to 4.0.0.
- [Release notes](https://github.com/rails/sprockets/releases)
- [Changelog](https://github.com/rails/sprockets/blob/master/CHANGELOG.md)
- [Commits](rails/sprockets@v3.7.0...v4.0.0)

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

Author: dependabot[bot]

docs/Gemfile.lock

@@ -17,7 +17,7 @@ GEM
     coffee-script-source (1.10.0)
     compass-import-once (1.0.5)
       sass (>= 3.2, < 3.5)
-    concurrent-ruby (1.0.2)
+    concurrent-ruby (1.1.5)
     contracts (0.13.0)
     dotenv (2.1.1)
     erubis (2.7.0)
@@ -89,15 +89,15 @@ GEM
     padrino-support (0.13.3.1)
       activesupport (>= 3.1)
     parallel (1.9.0)
-    rack (1.6.4)
+    rack (1.6.11)
     rb-fsevent (0.9.7)
     rb-inotify (0.9.7)
       ffi (>= 0.5.0)
     redcarpet (3.3.4)
     rouge (2.0.5)
     sass (3.4.22)
     servolux (0.12.0)
-    sprockets (3.7.0)
+    sprockets (4.0.0)
       concurrent-ruby (~> 1.0)
       rack (> 1, < 3)
     temple (0.8.2)