Update git submodules

Update git submodules

* Update docs/submodules/fuel from branch 'master'
  - [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>