doc: update user guide

This commit updates `user_guide.md` references and commands. It
also removes deprecated commands and includes new dependencies
used to build the firmware.

Instructions to run SCP for Total Compute (TC) are added to replace
System Guidance for Mobile (SGM) platforms.

Signed-off-by: Leandro Belli <leandro.belli@arm.com>
Change-Id: Ib11c7d3bd91078f76976fc72c73594a1d6b128fb