docs: improve REPL JSON documentation with examples and FAQ

[Mayank7Bisnewar]

This PR improves the Vix REPL documentation (docs/README_REPL.md) by expanding the JSON support section. It adds 5 concrete examples (Simple Objects, Arrays, Nested Data, Arrays of Objects, Mixed Types) and a troubleshooting FAQ table to address common syntax errors like missing colons or trailing commas.

Related issues N/A

Type of Change

1. Documentation update
2. Code Quality

• Code follows the project style and conventions
• C++20 (or newer) features are used appropriately
• No unnecessary complexity introduced
• No unused code, headers, or debug logs
• Public APIs are documented where applicable

Tests

• Existing tests pass locally
• New tests were added for new behavior or fixes
• Tests are located under tests/ or unittests/
• ctest --output-on-failure passes (Note: Pure documentation change, no code logic affected)
• Performance (if applicable)

Change has no negative performance impact
CLI / UX (if applicable)

CLI output is clear and consistent
Documentation

• Documentation was updated (README / docs)
• Examples were added or updated if relevant
• Comments explain non-obvious logic
• CMake / Build System

CMakeLists changes are minimal and scoped

Final Checks

• I have rebased on the latest dev branch

• Commit messages follow Conventional Commits

• This PR is ready for review

• Additional Notes Pure documentation update to assist users with strict JSON syntax in the REPL.