DOCS: Rename `django-idom` to `reactpy-django`

[Archmonger]

django-idom has rebranded itself to reactpy-django as of today. The docs should be updated to reflect this.

See here for traceability

• Add Django IDOM and package versions to comparison table #356
• Add Django IDOM to articles django-unicorn.com#21

[adamghill]

Thanks for the heads up. This has been updated. 👍