Bump concurrent-ruby from 1.3.6 to 1.3.7 in /docs/site

Bumps [concurrent-ruby](https://github.com/ruby-concurrency/concurrent-ruby) from 1.3.6 to 1.3.7.
- [Release notes](https://github.com/ruby-concurrency/concurrent-ruby/releases)
- [Changelog](https://github.com/ruby-concurrency/concurrent-ruby/blob/master/CHANGELOG.md)
- [Commits](ruby-concurrency/concurrent-ruby@v1.3.6...v1.3.7)

---
updated-dependencies:
- dependency-name: concurrent-ruby
  dependency-version: 1.3.7
  dependency-type: indirect
...

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

Author: dependabot[bot]

docs/site/Gemfile.lock

@@ -23,7 +23,7 @@ GEM
     bigdecimal (3.3.1)
     coderay (1.1.3)
     colorator (1.1.0)
-    concurrent-ruby (1.3.6)
+    concurrent-ruby (1.3.7)
     console (1.34.3)
       fiber-annotation
       fiber-local (~> 1.1)