Various improvements to Sidekiq documentation
Various improvements to Sidekiq documentation

- Made clearer the distinction between Sidekiq and govuk_sidekiq
- Describe how there is one shared Redis instance between machines
- Clarify Sidekiq Web port instructions
- Add an example Sidekiq console command for finding named workers

alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs,alphagov/govuk-developer-docs