Add ABAP File Format to serializer docs

[mbtools]

update https://docs.abapgit.org/development-guide/serializers/

• Split the section into "Classic File Format" and "ABAP File Format"
• Add recommended to prefer AFF for new serializer (like to AFF repo)
• Add code comments from Improve documentation of ZCL_ABAPGIT_OBJECT_COMMON_AFF abapGit#6601 to "AFF section
• ...