Section on Abjad

[behnam]

While working on #92, I realized that we never defined Abjad—the letter ordering and its numerical use.

I want to introduce the script-overview level of it to Chapter 2, so we can later use the concept for Counter Styles.

[behnam]

So, let's have sections on basics (terminology, encoding, direction, ordering) of Arabic Numerals and Abjad Numerals in Chapter 2.

Also we should cover Alphabetic ordering of letters in Chapter 2. Maybe mention CLDR as source of ordering encoded text.

Also, keep typography features and how numerals work with neighbor characters as a separate section in Chapter 3.

And, we get to typography side of Counters in Chapter 4. So, this section won't have definition of numerals or alphabet.