x/website/tour: toggle code editor off

[Deleplace]

Per #1437 and #62323, the Go Tour code box is not accessible to developers who are blind or visually impaired.

A useful mitigation would be to let the user choose to use the Tour in "read-only" mode:

• lesson text contents unchanged
• remove code editor
• replace it with a plain text listing
  • screen reader friendly
• keep the [Run] button
• remove the [Reset], [Format] buttons

This choice would persist in the user browser, like other existing settings.

[Deleplace]

As #62325 (link to the Playground) is most useful when we don't want to use the Tour's built-in editor, it may make sense to provide the link only in the new "read-only" mode.

Pros:

• no visual overhead in default mode
• less distraction/cognitive burden in default mode ("why do I have both a Run button and a Playground link? which one am I expected to use?)

Cons:

• Playground integration less discoverable

[Deleplace]

The need for a toggling away the editor would become somewhat obsolete if/when #1437 is resolved with a proper accessible code editor.