build: bump parallel from 1.18.0 to 1.19.0 in /docs

Bumps [parallel](https://github.com/grosser/parallel) from 1.18.0 to 1.19.0.
- [Release notes](https://github.com/grosser/parallel/releases)
- [Commits](https://github.com/grosser/parallel/compare/v1.18.0...v1.19.0

)

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

docs/Gemfile.lock

@@ -218,7 +218,7 @@ GEM
       mini_portile2 (~> 2.4.0)
     octokit (4.14.0)
       sawyer (~> 0.8.0, >= 0.5.3)
-    parallel (1.18.0)
+    parallel (1.19.0)
     pathutil (0.16.2)
       forwardable-extended (~> 2.6)
     public_suffix (3.1.1)