Docs should refer to Horizon / Meridian conditionally

The Welcome section of the Meridian docs includes the following static copy:

{blockquote}
The OpenNMS Group also offers Meridian, a subscription-based, more stable version of Meridian with long-term support.{blockquote}

For the case of the Meridian docs, this paragraph should instead talk about Horizon. We can fix this by wrapping the paragraph in an ifeval conditional.

The impact probably extends beyond the Welcome section.