Preview bot should use labels or other manual trigger by default

[jni]

The preview bot is a great idea, but it is very distracting to have it comment on each commit push to every PR, most of which are not touching the hub data at all (see e.g. here). I think the default mode should use a manual trigger, either with labels or with a command like @napari-hub-bot please preview.

Thoughts?

[neuromusic]

thanks @jni ... @Czaki has noted this too:

• Prewiew should not be rendered in unrelated pull requests #389
• Reduce number in comments added by preview action #448

now that we've wrapped up our 0.1.0 effort on napari-sphinx-theme, we're going to tackle this, initially by updating the comment in-place (see #422)