Add information about "alphabetic indexing"

[aphillips]

While adding text about sorting, it occurs to me that we need text about alphabetic index functionality, but I don't have time to write it now.

[xfq]

What's the difference between alphabetic indexing and regular human-visible collation?

[xfq]

Some CJK-related information:

• Pinyin alphabetical order
• Gojūon
• Korean

[aphillips]

"alphabetic indexing" is the part of human visible sorting in which sorted items are grouped together. A great example of this is the I18N Glossary, which has an (English) A-Z alphabetic index. Other languages use different alphabets with more or fewer letters; different scripts; etc. Content not in the user's language may still need to be grouped in various ways.