Documentation: update btrfs list of features and link to readthedocs.io

The btrfs documentation in kernel is only meant as a starting point, so
update the list of features and add link to btrfs.readthedocs.io page
that is most up-to-date. The wiki is still used but information is
migrated from there.

Signed-off-by: David Sterba <dsterba@suse.com>