ARROW-94: [Format] Expand list example to clarify null vs empty list

WIP to make sure what I've done so far looks good.  Per discussion on the JIRA item I started conversion of examples images to "text diagrams", but I wanted to get feedback to see if this is actually desirable (and if the way I'm approaching it is desirable).  The remaining diagrams are for unions which I can convert if the existing changes look OK to others (although I think the Union diagrams are are pretty reasonable/compact).

This change also includes some other minor cleanup, as well as including a statement about endianness per the discussion on the mailing list.

Rendered markdown can be seen at: https://github.com/emkornfield/arrow/blob/emk_doc_fixes_PR3/format/Layout.md

Author: Micah Kornfield <emkornfield@gmail.com>

Closes #58 from emkornfield/emk_doc_fixes_PR3 and squashes the following commits:

00b99ef [Micah Kornfield] remove png diagrams that are no longer used
cab6f87 [Micah Kornfield] a few more consistency fixes
5550a78 [Micah Kornfield] fix some off by one bugs
69e1a78 [Micah Kornfield] fix some alignment, and one last offset array to buffer conversion
b7aa7ea [Micah Kornfield] change list offset array to offset buffer
7dda5d5 [Micah Kornfield] clarify requirements of child types, finish replacing diagrams, fix some typos
0f23052 [Micah Kornfield] replace diagrams with physical layouts, clarify memory requirements for struct
590e4a7 [Micah Kornfield] cleanup magic quotes and clarify/fix some minor points