Switch Resource Groups docs "simultaneously" to "concurrently"

What does this MR do?

Slack discussion at https://gitlab.slack.com/archives/C3MAZRM8W/p1629388259368200

We use CI for deploying to GitLab.com and use resource_group to prevent multiple jobs from running concurrently. What we lack is the ability to prevent multiple pipelines from running concurrently (resource_group is at the job level). It looks like concurrency for actions https://docs.github.com/en/actions/reference/workflow-syntax-for-github-actions#concurrency can work on groups which is a bit nicer. There is some discussion about making this better for GitLab in #217522

Hacker news thread at https://news.ycombinator.com/item?id=28235351 with more context

The goal of this MR is to introduce the terms concurrency or concurrently to our Resource Groups documentation.

Pages affected: https://docs.gitlab.com/ee/ci/yaml/index.html#resource_group

Screenshots or Screencasts (strongly suggested)

How to setup and validate locally (strongly suggested)

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