Kaitai: `contrib/kaitai-struct-folder` reorganization

Context

Trivial MR

• contrib/kaitai-struct-files folder right now does mix .ksy .sh, .js , .MD and MAKEFILE files making it hard to navigate.
• Moreover, running make check-kaitai-struct-files or make validate-kaitai-struct-files complains if not everything inside contrib/kaitai-struct-files folder is not committed. In reality it should only complain if there are uncommitted changes for .ksy files.

We work around this problem by moving .ksy files to corresponding files/ folder and .sh and .js files to scripts/ folder. README.md has been updated accordingly.

Manually testing the MR

CI should pass

Checklist

• Document the interface of any function added or modified (see the coding guidelines)
• Document any change to the user interface, including configuration parameters (see node configuration)
• Provide automatic testing (see the testing guide).
• For new features and bug fixes, add an item in the appropriate changelog (docs/protocols/alpha.rst for the protocol and the environment, CHANGES.rst at the root of the repository for everything else).
• Select suitable reviewers using the Reviewers field below.
• Select as Assignee the next person who should take action on that MR