- Reorganized the document contents

- Reorganized the document contents
- Provided a "General Layout" section that currently covers just the block
  structure of the bytecode file.
- Wrote the section on the Global Type Pool
- Wrote the section on differences between LLVM file format versions.

Only five sections left to write!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@14628 91177308-0d34-0410-b5e6-96231b3b80d8