[components] Complete the components README (#27082)

## Summary & Motivation

This updates the main components tutorial/docs page:

- Adds significant explanatory material to the skeleton provided by the
README:
https://www.notion.so/dagster/Dagster-Components-README-14418b92e4628045a96bf54e88e3785d?pvs=4
- Covers ingest, transform, and automation
- Screenshots now included
- Updated to use `scaffold` instead of `generate`

## How I Tested These Changes

Manually (stepped through whole tutorial). I will add automated tests in
a follow-up, they are somewhat difficult due to the evolving
environment/structure of the project and need to download an external
DBT project repo etc.