Rewrite allo-todo to use stack layouts + update getting started guide accordingly

[failuer]

Resolve the stack layouts

Either one of:

• Rewrite stacks so that they layout correctly even when two vertical stacks are nested
  or
• Simplify the todo app so it only uses a single vertical stack

Coding the Todo app

• Finish the stack-based layout instead of an absolute-positioned layout

Update docs.alloverse.com > Getting started

• Add the new allo-todo snippets to the guide
• Update the explanatory text
• Update the screenshots (use the dev sandbox environment)

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Original "parent" issue: alloverse/open-planning#94

[failuer]

Blocked by not knowing how to best move forward with the LayoutStacks - nesting two vertical stacks causes them to not layout as expected.