Remove old documents

[aiotter]

Removes old {:push, graph :: Graph.t()} option from documentations.

Description

{:push, graph :: Graph.t()} is removed from Scenic.Scene.response_opts() when it went to v0.11.
However the current document still insists that you can return it from Scenic.Scene.init() callback.

Motivation and Context

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to
  not work as expected)
• Improvement/refactoring (non-breaking change that doesn't add any feature
  but make things better)

Checklist

• Check other PRs and make sure that the changes are not done yet.