docs(walkthrough): add 'Building contest statements' walkthrough (Delivering track, step 3)

[rsalesc]

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

Scope

New page under Walkthrough → Delivering a contest, step 3. Narrative end-to-end of building the contest-level statement PDF.

Cover:

• rbx contest statements build.
• Statement groups, multi-language statements, contest cover, problem-letter assignment.
• Customizing the contest template (link to the rbxTeX feature-guide pages for deeper reference).
• Common pitfalls (missing per-problem statements, language mismatches, asset paths).

Recording

• Screenshot of the resulting contest PDF (front cover + a problem page).