Documentation for hostname-override-source is missing

**Path to page with content (_do not_ include domain name):**

**What content is incorrect?**
Documentation for `hostname-override-source` is missing. It was made available in 1.21.0. Feature PR [link](https://github.com/bottlerocket-os/bottlerocket-core-kit/pull/59)

**What should the content say or communicate?**
