Expand sections on Diacratics

[behnam]

We cover the basics for Arabic Diacritics in Chapter 2: https://w3c.github.io/alreq/#h_diacritics

We need to expand the Encoding side of it better (user text input vs NFC?) and also mention the new UTR #53 spec.

On Chapter 3, we need a new section for Diacritics to get to the typography features, like having different style (colors), or special positioning.