User guide alignment

[AnthoJack]

There seems to be some out-of-date info in the documentation's user guide

• The default example in the user guide migrated from vexpress to virt32 some time ago but the user guide still uses vexpress as platform in "Creating the virtual disk image"
• In "Compiling the kernel space", the config used is "virt32_ramfs_defconfig" which doesn't exist anymore
• PLATFORM in build.conf is "virt64" by default. Maybe it should be "virt32" to be more coherent with user guide or the user guide should be updated to use virt64 as default
• The docker part of the user guide is unfinished :
  • Uses vexpress as default example instead of virt32/64
  • The 2 last sentences suggest some command line examples should follow but nothing is there
  • Both drun and drunit launch interactive containers