Moving OPM demo songs to their own folder, and adding a small additional info in PowerNoise editor (#2366)

* OPM demos should have their own folder

Five OPM demos now, at this point they should have their own folder.

* Delete demos/misc/the_king_of_crisp_opm.fur

* Delete demos/misc/vortex_opm.fur

* Delete demos/misc/waterworld_map_opm.fur

* Delete demos/misc/lagrange_point_2023_opm.fur

* Delete demos/misc/hope_for_the_dream_opm.fur

* Update powernoise.md

I believe this clarification is necessary in case people initially get confused about where the AM exactly is outputted to.

doc/4-instrument/powernoise.md

@@ -33,7 +33,7 @@ by default the LFSR is configured to produce square waves, by having a single ta
 - **Pitch**: fine pitch.
 - **Phase Reset**: trigger reloading the LFSR.
 - **Control**: channel settings:
-  - **slope AM**: when enabled, this channel's output and the slope channel go through amplitude modulation.
+  - **slope AM**: when enabled, this channel's output and the slope channel go through amplitude modulation. The final output plays in the slope channel.
   - **tap B**: enables use of two taps for the LFSR.
 - **Tap A Location**: sets the position of the first tap.
 - **Tap B Location**: sets the position of the second tap.