Convention for file names of API definition

[rartych]

Problem description
There is currently no guidelines for file names of API definition files in CAMARA.
The initially agreed proposal is to use the convention api-name.yaml as a guideline for the API definition file names.

Expected behavior
Change the file name to network-access-management.yaml before or as a part of release PR for M4.

Alternative solution
Wait for the final guidelines.

Additional context
See the discussion in Issue in Commonalities

[hdamker]

@rartych @caubut-charter NetworkAccessManagement hasn't had a M3 release candidate, so it can still be done as part of #17