[license] Cleanup duplicate LICENSE files

[license] Cleanup duplicate LICENSE files

github.com auto-detects the licensing terms if and only if the root
directory contains a single license file.

To leverage this, distinguish via main README file between:
- main fuel repository license (apache-2.0);
- documentation license (cc-by-4.0);
- 3rd party licenses (non-exhaustive list for 3rd party projects);

While at it, add missing license headers to docs auxiliary files.

Change-Id: I7bd25573b0c2b8d16881c7ae72198f5981683c08
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>