Improve discoverability of Plugin Previews

[akirk]

This modifies a few bits in the plugin directory to make it easier to discover plugin previews and how they work:

Add a "Preview Not Available"

To indicate that there could be a preview button. The eye indicates that this is only visible to the plugin admin.

Before:

After:

Link the documentation on the advanced page

Before:

After:

Add an eye to the "Test Preview"

Indicate that the "Test Preview" doesn't mean the preview is enabled:
Before:

After:

[StevenDufresne]

Add a "Preview Not Available

I don't know either way whether that would work, but we can always run the experiment.

Link the documentation on the advanced page

• We should change that logic so an error banner is not shown unless the blueprint is broken.
• I would also consider updating the language to do a bit of selling: "Live Previews allow users to experience your plugin in real-time. Read more about Live Previews."