Add document about page grouping functions

I wrote some documents about grouping functions added at #425. I added them to end of the template/list document, near the order functions descriptions.

While I was writing them, I came up with some improvements of the functions

- Changing Data property name of PageGroup to Pages is more user friendly
- It is more consistent using Reverse function to get reverse ordered PageGroup array as Pages does instead of using order argument ("asc" and "desc") at those function calls
- I couldn't think any use cases except using with .Section, .Type, .Date, .PublishDate (Someone may also need .Keywords and .Indexes). So, the current GroupBy implement may be too much flexible. It may be better to rewrite it some functions like GroupByType, GroupBySection etc.

Maybe it should be discussed as other issue. If so, I'll create an issue.