cassini-doc: Refactor Cassini documentation

The Cassini documentation is refactored to separate user and developer
manuals. The `Build, Deploy and validate Cassini Image` document is
moved to user manual section.

User manual:
    * Build, Deploy and validate Cassini Image

Following new documents are added to developer manual section.

Developer manual:
    * User Accounts
    * Build System
    * Yocto Layers
    * Security Hardening
    * Software Development Kit (SDK)
    * Validation

Signed-off-by: Devaraj Ranganna <devaraj.ranganna@arm.com>