Document HA scheduler

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**

Ballista can support HA scheduling, it used to support it out of the box in the past, but not at the moment. Some companies have implemented it.

**Describe the solution you'd like**

Add few pages in documentation outlining process needed to support HA scheduler

**Describe alternatives you've considered**

**Additional context**

Start by reading the existing Ballista documentation on scheduler deployment and the repository's documentation structure. The issue asks for pages outlining the process needed to support a high-availability scheduler; identify the prior built-in support and approaches companies have implemented before drafting. Done means the documentation clearly explains the HA scheduler process; the issue does not name specific files or tests.

Written by the indexing model from the issue text.