Update man page documentation to remove CNI references

Removed all CNI-specific documentation from man pages:

- podman.1.md: Simplified --network-config-dir to only mention
  netavark directories
- podman-network.1.md: Removed dual backend description, now states
  netavark is the only backend
- podman-network-create.1.md: Removed CNI-specific notes about DNS
  and DHCP socket configuration
- podman-network-connect.1.md: Removed CNI limitation note about
  network aliases
- options/network-alias.md: Removed CNI limitation note about
  network aliases
- podman-info.1.md: Updated example output to show netavark backend
  information instead of CNI

All man pages now reflect netavark as the sole network backend.

Signed-off-by: Lokesh Mandvekar <lsm5@redhat.com>