docs: audit menus and item builder pages

[demengc]

Summary

• Fix createCommandHandler() usage in menus.md to include the required .build() call (it returns Lamp.Builder, not Lamp)
• Restructure both pages with dense reference tables for all API methods, replacing scattered code-only snippets
• Correct the glow effect example in item-builder.md to remove redundant flags(ItemFlag.HIDE_ENCHANTS) (already applied internally by glow(true))
• Document all 5 MenuButton.create() overloads with slot-handling semantics
• Add inventory slot grid diagram and indexing table clarifying 0-based (code) vs 1-based (config) for slots, rows, and columns
• Document copy() shallow reference semantics, addStaticButton(), applyFillersFromConfig(), onClose callback, and static material utilities

Test plan

• Verify markdown renders correctly on GitBook
• Confirm all code examples compile against the current API
• Check that table formatting displays properly in GitBook