Prepare CHANGES.md for 5.2

[MaelRL]

Summary of Changes

Preview: https://github.com/MaelRL/cgal/blob/CGAL-Prepare_changes.md-5.2-GF/Installation/CHANGES.md

@efifogel CHANGES.md say that CGAL::Object is replaced by boost::variant, but in decompose(), the template parameter still uses CGAL::Object; is that normal?

Release Management

• Affected package(s): Installation
• Issue(s) solved (if any): -
• Feature/Small Feature (if any): -
• License and copyright ownership: -

[lrineau]

I have checked that the links are good. At least the path part. For the fragment parts (after #) I do not know if there exists a link checker that checks fragments as well.