Update FAQ, upgrade guides for v2.3

[joeyparrish]

As we close out v2.3, the FAQ and upgrade guides need to be updated. Many things have changed since v2.2.

[joeyparrish]

Checklist:

• Update FAQ
• Update architecture diagrams
• Update config tutorial to reflect new options in v2.3 and some v2.2.x updates
• Update change log for v2.3
• Write a new upgrade guide for v2.2 => v2.3
• Update offline tutorial WRT multiple downloads in parallel (Concurrency Issues with Multiple Storage Instances #1047)
• Reread the plugin interfaces tutorial, update if needed
• Reread and execute the manifest parser tutorial, update if needed
• Update service worker tutorial with regard to best practices
• Update existing upgrade guides to target v2.3

[vaage]

@joeyparrish Do you want the parallel downloads tutorial as part of 2.3 or 2.4? I slotted the issue for the tutorial for 2.4.

[joeyparrish]

I will update the comment in the existing tutorial to note that parallel downloads are possible, but I will defer fully updating the tutorial to demonstrate that (#1196).