photon-os-installer: Adding docs and sample_ks directory

[jaankit]

• Moved ks_config.txt to docs/ks_config.md
• Moved ui_config.txt to docs/ui_config.md
• Moved sample_ks.cfg sample_ks/sample_ks.cfg
• Moved sample_ui.cfg sample_ks/sample_ui.cfg
• Added various sample_ks files under sample_ks/ for different features such as:
  • ostree
  • preinstall, postinstall
  • default configs

Change-Id: I00e5fe3bd56d4cea04e7378663b8365f8400ab1e