Remove documentation for SLES

[pleia2]

SUSE Linux Enterprise Server 15 has been out for years now, but our documentation does not include it.

Test a SLES 15 install, and add it to our documentation.

[pleia2]

It turns out that we don't have any deployments using SLES as the base system, and we don't have the resources to maintain the SLES documentation in addition to Ubuntu, which we use in production.

Changing the issue to be "Remove documentation for SLES" at this time.

Contributions welcome if there's a desire to restore it and keep it maintained.