Re-organize VIXL's doc/ and mention AArch32

Updates the top README to reflect changes done with the support for
AArch32. Take the opportunity to re-organize our documentation files
structure and seperate `aarch32` and `aarch64` at the top-level.

Additionally, the title now mentions this is the
development version of VIXL.

doc/
├── aarch32
│   ├── design
│   │   ├── code-generation-aarch32.md
│   │   └── literal-pool-aarch32.md
│   └── getting-started-aarch32.md
├── aarch64
│   ├── getting-started-aarch64.md
│   ├── supported-instructions-aarch64.md
│   └── topics
│       ├── extending-the-disassembler.md
│       ├── index.md
│       └── ycm.md
└── changelog.md

Change-Id: Ida9547a1de30886a112360e4b2beafa7cb0ac7b9