Document ElementNode API

[KSAGlory]

Summary

• document every method declared by ElementNode, including traversal, mutation, selection, copying, and layout behavior
• add a DocC page that groups the public API by task
• preserve all existing runtime behavior

Validation

• git diff --check
• verified all 36 symbols linked from the DocC page exist in ElementNode.swift
• reviewed the final diff to confirm it contains documentation changes only

Environment note

The DocC build was not run locally because the current environment is Windows and does not provide Xcode. The documentation follows the existing Node.md structure and Swift DocC symbol-link conventions.

Fixes #22