Align license file handling with FMI

[Tc-Fast]

Currently SSP does not specify where to put license files.

FMI on the other hand does specify the use of license files:
model.fmu
-- modelDescription.xml
-- documentation
---- index.html // entry point of the documentation
---- ...
------- licenses // directory for licenses (optional)
---------- license.{txt|html|spdx} // entry point for license information
---------- <license files> // for example BSD licenses (optional)
...

Adding it to SSP seems easy:
diagram.ssp
-- SystemStructureDescription.ssd
-- documentation
---- index.html // entry point of the documentation
---- ...
------- licenses // directory for licenses (optional) <--- NEW
---------- license.{txt|html|spdx} // entry point for license information <--- NEW
---------- <license files> // for example BSD licenses (optional) <--- NEW
...