Where can I find official documentation for install Oracle 11g r2 XE on linux?

And you said the US got the better ones?

It was my understanding they were technically the same as a Fiat 128, Yugo having purchased the plans and tooling from Fiat.

1. To determine the free disk space on each mounted file system, use the following command: # df -h
   
   • 
     

     [oracle@vmoraclelinux ~]$ df -h
     Filesystem       Size       Used       Use%       Mounted on
     /dev/sda3         36G       3.6G        11%       /
     tmpfs            935M       304K         1%       /dev/shm
     /dev/sda1        291M        39M        14%       /boot
     [oracle@vmoraclelinux ~]$
     

2. From the display, identify a file system that has the appropriate amount of free space
   
   • from the above point (1.) it look like that the file system that has the appropriate amount of free space is the follow:
     

     Filesystem       Size       Used       Use%       Mounted on
     /dev/sda3         36G       3.6G        11%       /
     

3. Note the name of the mount point directory for the file system that you identified.
   
   • from the point 2. the mount point directory is the root directory (/)

1. Enter commands similar to the following to create the recommended subdirectories in the mount point directory that you identified and set the appropriate owner, group, and permissions on them:

Just follow the advice:

# mkdir -p /u01/app/oracle
# chown -R oracle:oinstall /u01/app/oracle
# chmod -R 775 /u01/app/oracle

According to me, as far as I understood, Oracle wants that we create a partition on disk, which mount point directory is u01, for install oracle software correctly.

Is it right?

You already have a partition. It is mounted to / (the root directory). In a more sophisticated system you might want to create a partition dedictated to oracle, but you do not need to do that and doing so is an un-necessary complication for your needs. Just create the directories as per the example and as per re-inforced by John.

Since you seem to be doing the specifically to learn how to manage oracle on linux, you really, really, really need to take some time learning some fundamentals of linux. In this case, you need to learn about mount points, directory names, and mounting a directory.

And you need to be doing it on a version of Linux that is specifically supported by Oracle .. not one that you think should be supported by Oracle just because it is similar to one that is.