Add concurrency limit recommendation to Sidekiq dev docs

What does this MR do and why?

This MR adds a section about Concurrency Limit Middleware to the Sidekiq development documentation.

It also adds a recommendation to set it for all workers.

Rendered version: https://gitlab.com/gitlab-org/gitlab/-/blob/add-concurrency-limit-to-sidekiq-dev-docs/doc/development/sidekiq/index.md#concurrency-limit

https://gitlab.com/gitlab-org/gitlab/-/issues/477189

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