Extend the documentation
This MR extends the documentation in the following ways:
- enumerating the required capabilities
- showing a short description of each flag/option
- porting the documentation from the C version on how to do xattr mapping
- adding links to relevant sections/documents, such as contributing, running as non-root and xattr mapping
- adding the uid/gid mapping requirement for user namespaces
- adding a shot FAQ section
It also adds the off
option to the --log-level
documentation and changes --sandbox none
to --sandbox chroot
in the "Running as non-privileged user" section examples.