Update Asset Pipeline Guide to describe how Sprockets 4 works
Update Asset Pipeline Guide to describe how Sprockets 4 works

Fixes https://github.com/rails/rails/issues/43203

To test this, I
- Uninstalled all Rails gems
- `gem install rails --pre`
- `rails new asset_test`
- Made these changes, to use Sprockets for JS instead of importmaps: https://github.com/ghiculescu/assets_test/commit/7d961765c0c1b487d6cac20584707089ac6932bf
- Confirmed that no source map is rendered by default
- Set `config.assets.debug = true`, and confirmed that a source map is now rendered
Erol/rails,Erol/rails,yawboakye/rails,kmcphillips/rails,Edouard-chin/rails,BlakeWilliams/rails,yahonda/rails,Vasfed/rails,rails/rails,georgeclaghorn/rails,esparta/rails,kmcphillips/rails,esparta/rails,kddeisz/rails,rails/rails,Vasfed/rails,rails/rails,Edouard-chin/rails,BlakeWilliams/rails,yahonda/rails,kddeisz/rails,BlakeWilliams/rails,yawboakye/rails,yawboakye/rails,georgeclaghorn/rails,MSP-Greg/rails,BlakeWilliams/rails,MSP-Greg/rails,esparta/rails,yahonda/rails,Vasfed/rails,Erol/rails,georgeclaghorn/rails,MSP-Greg/rails,kddeisz/rails,georgeclaghorn/rails,MSP-Greg/rails,Edouard-chin/rails,kmcphillips/rails,kmcphillips/rails,esparta/rails,Vasfed/rails,Edouard-chin/rails,Erol/rails,rails/rails,kddeisz/rails,yawboakye/rails,yahonda/rails