docs: Document `turbo.*` generator variables (#12511)
## Summary
- Documents the `turbo` object that Turborepo injects into generator
action answers (`turbo.paths` and `turbo.configs`), which was previously
only discoverable by reading source code.
- Adds a guide-level section to the [Generating
code](/docs/guides/generating-code) page showing usage in both
Handlebars templates and custom actions.
- Adds a full reference table to the
[`@turbo/gen`](/docs/reference/turbo-gen) page.
Closes #8708
