Add a disclaimer about CentOS for YUM install (#15453)
* update step 3
* fix formatting
* fix formatting
* fix formatting
* test formatting
* change code to use sphinx directives
* fix formatting
* test formatting
* test formatting
* rewording
* test formatting
* test formatting
* test formatting
* move content to troubleshooting guide
* fix formatting
* fixes
* remove troubleshooting item and add a disclaimer
* add centos 7
