oob: Fix OOB findings

* Install missing `libXcursor` package needed by Fast-models third
party add-ons installer
* Install missing `libncursesw5` package needed by arm-none-eabi-gdb
* Install ninja build tool
* Fix incorrect link for `container_post_startup.sh` in
devcontainer.json
* Improve documentation based on OOB feedback

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