docs: add item descriptor guide

[Zodiarche]

Summary

Adds an item descriptor guide as the second page of the new Data section (EN + FR), migrated from # Describe Item.md in the engine repository. The page explains how PFM::ItemDescriptor turns Studio::Item data into in-game behavior: the wrapper read by the UIs, the 5-step usage flow, and every define_* function with a real vanilla example verified against the engine source (Sacred Ash, Repel, Gracidea, X items, PP Up, Bicycle), plus a custom item built from A to Z.

Compared to the original .25-era document: the renamed define_on_creature_* API, current Studio::* examples instead of GameData::*, the battle side fully exemplified, and previously undocumented behaviors (automatic chen from the Studio flags, :__undef__ and no_effect, is_limited consumption, targeting precedence, enforced definition order, chen vs :unused decision criteria).

Stacked on #39: this branch is based on docs/data-access-guide because the psdk/data section is created there. Merge #39 first; GitHub will retarget this PR to main automatically.

Related issues

Closes #40

Test plan

• bun run typecheck passes.
• bun run build passes on both locales (validates the cross-links with the data access guide, locale-specific slugs).
• Every code example cross-checked against the engine source (000 ItemDescriptor.rb, 001 RepelItem.rb, 300 StatBoostItem.rb, 300 PPIncreaseItem.rb, 001 EventItem.rb).
• Visual check to do during review: page renders at /psdk/data/item-descriptor (EN) and /fr/psdk/data/definir-le-comportement-d-un-objet (FR), right after the data access page in the sidebar.

Screenshots

No image on this page (the source document had none).