(fix) controls: fix / update scratch2 and jog, remove scratch

[ronso0]

Also replaces the (for users) rather useless link to mixxxdj/mixxx#5350 with a link to the jog wheel script example.

Preview:
https://deploy-preview-694--mixxx-manual.netlify.app/chapters/appendix/mixxx_controls#control-[ChannelN]-scratch2

---

Btw I'm wondering why the gh-wiki-anchors hook insists on changing links
from
https://github.com/mixxxdj/mixxx/wiki/midi%20scripting#scratching-and-jog-wheels
to
https://github.com/mixxxdj/mixxx/wiki/midi%20scripting#user-content-scratching-and-jog-wheels

[Holzhaus]

I suggest to add the content to the manual instead of linking to the wiki. The wiki is not really versioned in the sense that if we overhaul jog wheel handling in Mixxx version X, the wiki content will probably change and old manual versions (< X) will thn point to wiki content that does not apply to that version.

[ronso0]

Hmkay, I understand.
What place in the manual do you think would suit best?

FWIW we also link to the MIDI page in https://manual.mixxx.org/2.4/chapters/advanced_topics#midi-scripting for example.