Topic/f update (#364)

* Update f-digital-credential-api.md

Textual changes to comply with established terminology in the ARF:
Wallet Solution -> Wallet Unit
credential ->attestation
shall -> SHALL

Also fixing some typos.

* NI-Scy internal review

* update in section 2.2

* fix typo  in section 2.2

* Added problem statement and minor fixes on formating

* raw requirements to be discussed internally

* blank lines removed

* updated based on our internal review

* minor modification in privacy preservation section

* fix quotes

* fix quotes

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Co-authored-by: David Bakker <75264147+david-bakker@users.noreply.github.com>
Co-authored-by: Nikos Fotiou <nikosft@gmail.com>
Co-authored-by: Paul Hin <phin@scytales.com>

Author: 4 people