Editorial improvements

[peppelinux]

I want to merge the following macrosections

7. Registry Infrastructure
8. Entity Onboarding
9. X.509 Certificate Management Operations

in preexistent sections, in the following way:

1. X.509 Certificate Management Operations and Registry Infrastructure as subsections and linked annexes about trust infrastructure.
2. Entity Onboarding in onboarding service

I'd go with this improvement as the first PR within the miletone 1.6.0