Documentation: update the Squashfs filesystem documentation

This patch updates the following which are out of date.

- Zstd has been added to the compression algorithms supported.
- The filesystem mailing list (for the kernel code) is changed to
  linux-fsdevel rather than the now very little used Sourceforge
  mailing list.
- The Squashfs website has been changed to the Squashfs-tools github
  repository.
- The fact that Squashfs-tools is likely packaged by the linux
  distribution is mentioned.

Link: https://lkml.kernel.org/r/20241229233752.54481-4-phillip@squashfs.org.uk


Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>