[CI:DOCS] rootless_tutorial: Remove incorrect advice regarding volumes and mount points

* Remove the instruction
  "The mount point must exist in the container"
  as it is incorrect which is shown by this example
  ```
  $ mkdir dir1
  $ podman run --rm -v ./dir1:/dir1:Z docker.io/library/fedora:36 touch /dir1/file
  $ ls dir1
  file
  $ podman run --rm docker.io/library/fedora:36 ls -l /dir1
  ls: cannot access '/dir1': No such file or directory
  $ podman --version
  podman version 4.1.1
  $
  ```

* Rewrite the advice "You should always give the full path to the volume you'd like to mount"
  so that it also mentions relative paths starting with a dot.

Signed-off-by: Erik Sjölund <erik.sjolund@gmail.com>

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