* [Guide] ES6: Prevent object mutation; fixes and enhancements
- Remove phantom image
- Remove notes for contributors
- Change explanation and hints.
- Links to markdown
* Changed as requested
Changed titles as requested by @randelldawson.
Note: I'm just used to [MDN](https://developer.mozilla.org/en-US/docs/MDN/Contribute/Guidelines/Writing_style_guide#Page_titles) and other language styles and though it was clearer. Is it ok now for this guide?
* fix: removed repl.it link
