docs(walkthrough): refactor packaging-walkthrough.md as 'Packaging a problem' (Delivering track, step 4)

[rsalesc]

Part of the Walkthrough restructure (see docs/plans/2026-05-07-walkthrough-restructure-design.md).

Scope

Refactor docs/setters/packaging-walkthrough.md to fit its new role as step 4 of the Delivering-a-contest track.

Changes:

• Remove the Step 1: Profile the time limit section (now lives in the dedicated profiling walkthrough — separate issue). Replace with a short prereq callout that links there.
• Reframe headings from Step 1 / 2 / 3 (within the page) to align with the track-level numbering.
• Keep BOCA as the primary target, but tighten sidebar links to Polygon / MOJ / PKG so readers know parallel paths exist.
• Keep automated-upload section.
• Update intro/outro cross-references to point at the new track neighbors (profiling before, contest packaging after).

Out of scope

• Authoring new content for Polygon/MOJ/PKG.
• Changes to the rbx package command itself.