[Build Todo] Use complete replacement `zilla.json` instead of delta changes

[jfallows]

When folks are learning zilla via Build the Todo Application guide, they do not yet fully understand how the zilla.json format works, so it is unnecessarily complex to ask them to precisely edit zilla.json to apply a change.

Instead we can describe the change in the guide, but provide a complete replacement zilla.json, increasing the likelihood of them being able to successfully complete the guide.

This is based on feedback from @ralyodio.

[jfallows]

Docs updated with Zilla Studio screenshot, brief description and complete zilla.json replacements instead of deltas. See https://docs.aklivity.io/zilla/get-started