Format React handbook: normalize quotes & layout

Reformat React interview handbook files (part1–part4) and add _config.yml. This commit standardizes code style (double quotes in code, consistent arrow/function spacing, semicolons where appropriate), normalizes Markdown tables and lists for better alignment, tidies JSX examples, fixes small formatting/cleanup issues in hooks examples (useEffect cleanup, useState/useCallback usages), and improves readability of many code blocks and prose. Also adds a site config (_config.yml). No functional logic changes — just stylistic and docs improvements.

Author: adityaSrivastava29