[Virtio-fs] [virtiofsd] MR opened: 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.
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https://gitlab.com/virtio-fs/virtiofsd/-/merge_requests/97