I want to start a Discussion about a topic on a page that doesn't already have a Discussion. I've realized that some pages already have a Discussion, and above it is a "Start Discussion" button. I can't seem to find this on pages without an already-existing Discussion, though. How do I start a Discussion on a page without one already existing?
The page you linked to has Article Comments which, despite what Dunnoob's comment may imply (intentionally or otherwise), should be available on all desktop skins, not just the default one. Depending on the wiki, this feature may or may not be enabled. If it is, the top of the comments section will provide everything necessary to start a "discussion". If you want to reply to a comment, click the reply button on that comment. If you are using the mobile skin, you can only view comments. At the bottom of the comments section, you will see a button to start a new "discussion" but this will force your device to use the default desktop skin. As far as I can tell, you cannot reply to comments using the mobile skin.
As for talk pages, wikis typically only use these if comments are not enabled.