Rewrite Sidekiq processes docs

The existing docs focus too much on what you can do, not what you should do. Let's reorder and rewrite them to fix that, with a clear recommendation to not set up processes listening to specific queues.

Part of gitlab-com/gl-infra/scalability#1991 (closed).

TOC before:

• Available Sidekiq queues
• Start multiple processes
• Negate settings
• Queue selector
• Ignore all import queues
• Number of threads
  • Manage thread counts explicitly
    • When running Sidekiq cluster (default)
    • When running a single Sidekiq process
• Modify the check interval
• Troubleshoot using the CLI
  • Monitor the sidekiq-cluster command
  • PID files
  • Environment

After:

• Start multiple processes
• Concurrency
  • Manage thread counts explicitly
• Modify the check interval
• Troubleshoot using the CLI
  • Monitor the sidekiq-cluster command
  • PID files
  • Environment
• ⚠ Processing specific job classes
  • Routing rules
  • Queue selectors
    • Negate settings