Shift qa-sandbox <CodeExample> markers to 0-based (follow-up to skola.dev#202)

[joaquimscosta]

Context

skola.dev PR #202 makes <CodeExample position={n}> 0-based, consistent with <Quiz> and the stored position (previously it was 1-based, which silently broke the intro-python examples authored as position={0}).

The fix inverts which markers are correct:

• intro-python used position={0} → now correct, no MDX change needed.
• qa-sandbox used position={1}/{2} → now off-by-one, renders nothing until shifted down by one.

qa-sandbox is a sandbox/showcase course (not learner-facing), so this is low-urgency, but the examples are currently blank.

Changes needed (qa-sandbox MDX markers)

Lesson	Current	Change to
qa-sandbox/lessons/03-mixed-everything/kea.mdx	<CodeExample position={1} />	position={0}
qa-sandbox/lessons/04-callouts-and-figure/kea.mdx	<CodeExample position={1} />	position={0}
qa-sandbox/lessons/05-code-notations/kea.mdx	<CodeExample position={1} />, position={2}	position={0}, position={1}

Also (intro-python — comment cleanup only, no functional change)

The 3 code-examples.yaml sidecars in intro-python (L02/L07/L09) carry a now-misleading comment: "position 0 corresponds to <CodeExample position={0}>". With the fix this is finally accurate, but the comment was written under the old (broken) assumption — worth a quick reword to "0-based marker = stored 0-based position" for clarity.

After editing

Republish + task content:bump per the content pipeline. A redeploy is required for the static lesson routes to pick up the MDX change.

Guardrail

PR #202 adds a dev console.warn when a <CodeExample> position matches no example, so this class of drift is no longer silent during authoring.