Add details on installing plug-in versions

Suggestion from the [July contributors’ call](https://github.com/dita-ot/dita-ot/wiki/Meeting-minutes-2024-07-11#item-3-doc-updates-and-plans):

> we have a `dita install` command to install plugins, docs could use an update to describe how to install a specific version of the plugin. Just giving the plugin name gets you the latest.

