Conditionally Remove Irrelevant Sections from Plugin Manual

[ryukau]

When a plugin has no presets, it seems confusing to put instructions of preset installation on the manual. These irrelevant sections should be removed.

This issue is made as a reminder. Priority is low. Context can be found in the comment section of the link below.

• MembraneSynth Demo - YouTube

[ryukau]

I took a shortcut at adcd541. It added a notice that some plugins don't have presets.

I keep open this issue, because it can be done better.