Update ChestShop player/admin docs: partial trades + native Nexo custom items

Clarify changed/rewrite ChestShop behaviour and document the native Nexo integration (PAR-291) in the economy-explorer docs.

• Partial trades: the guides/reference/FAQ said a short side "pauses until topped up" (implying all-or-nothing). Rewrote to describe actual behaviour (ALLOW_PARTIAL_TRANSACTIONS): a trade serves as much as it can and charges/pays pro-rata, stopping only at zero. New "Partial trades" section in chestshop-basics, a note on line 4 in the reference, and a corrected FAQ answer.
• Custom items (Nexo): new "Selling custom items" section in chestshop-basics (hold item + ?, or type the bare id / /iteminfo; stock the real item — exact-match only), a line-4 mention + link in the reference, a new FAQ entry, and a "Custom items (Nexo)" admin section documenting nexo.yml (aliases/display/input) — notably why fallbackFormat: %s (bare id) + acceptBareIds are required (ChestShop's item-line pattern rejects colons, so nexo:<id> fails sign validation). Also noted as a fork behaviour (support compiled in; no companion plugin).

Touched: guides/chestshop-basics.md, reference/chestshop.md, admin/config-chestshop.md, faq.md.