docs: update specifications

[javier-godoy]

Summary

• Three-level configuration tree — documents the Column → Instance → Global resolution order with scope-first traversal, replacing the fictional EasyGrid.registerTypeConfig stub with the real GlobalEasyGridConfiguration / EasyGrid.forType API.
• Null representation — specifies the default ("" from the global Object config) and how to override it at any level.
• Type hierarchy support — documents that each scope level walks the class hierarchy, with primitive-to-boxed mapping, so a Number.class config is inherited by all numeric subtypes.
• Renderer utility classes — documents LocalDateRenderers, LocalDateTimeRenderers, and NumberRenderers with their available overloads.
• Boolean rendering — clarifies that "true"/"false" are string literals from Boolean.toString() and that any other representation requires an explicit custom formatter (i18n).
• Enum rendering — corrects the table entry from name to toString() and adds a note citing the Java documentation rationale.

[mlopezFC]

Just one comment, maybe I'm missing something

[mlopezFC]

LGTM