Document the rendering life cycle

### Location

A new Architecture section of the docs: https://github.com/google/blockly/issues/7236

### Type

- [X] Text
- [X] Image or Gif
- [X] Other

### Suggested content

Write up a doc explaining the rendering order (both for immediate rendering and queued rendering). Because right now the logic lives in lots of files and requires a lot of context.

### Additional context

N/A