doc: Move platform specific instructions to `Getting Started` guides

Build, deployment and reset instructions vary between N1SDP,
Corstone-1000 for MPS3, and Corstone-1000 FVP so they should be in one
of the platform specific documents (`Getting Started with...`).

Conversely, login instructions are platform independent and should be
moved from the platform specific documents to the generic user guide.

A `Getting Started with...` document is created for the N1SDP and all
platform specific instructions in the generic user guide are moved to
the relevant platform guide (N1SDP, Corstone-1000 MPS3, or
Corstone-1000 FVP).

Each `Getting Started with...` document starts with a section which:
- names the platform kas config file
- shows the command used to build a base image for that platform
- shows the image files created (firmware and distro)
The structure of this section is aligned across all supported platforms

The generic build instructions in the user guide now contains links to
each of the platform specific `Getting Started` guides.

Changelog: other
Signed-off-by: Adam Johnston <adam.johnston@arm.com>