Install Gems to vendor/bundle

What does this MR do and why?

Installs Ruby gems to vendor/bundle so that they can be cached with our existing caching settings.

Docs: https://docs.gitlab.com/ee/ci/caching/#cache-ruby-dependencies

Closes #1677 (closed)

Screenshots, screen recordings, or links to review app

• Pipeline that ran .bundle_and_yarn: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/4649297339#L3730
• Pipeline than ran .bundle: https://gitlab.com/gitlab-org/gitlab-docs/-/jobs/4649297355#L181

Merge request acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this merge request.

Closes #1677 (closed)