docs: Add Stem VU Meter controls to appendix

[xARSENICx]

Goes with PR.

This is my first time writing manual doc for a feature, so I apologize for any lack of foresight

[JoergAtGithub]

Please target this to main branch, same as the PR with the code

[xARSENICx]

Please target this to main branch, same as the PR with the code

Ok. Does this look fine to you? (The number of file changes got me worried 😓 )

[JoergAtGithub]

You only changed the target of the PR on the GitHub website. You need also to rebase your git branch.

[acolombier]

Previously, the decision was made to document [ChannelN_StemM] COs alongside other channel COs ([ChannelX], [SamplerX] or [PreviewDeckX]). Either those new CO should be brought in the same way, alongside the other existing vu_meter_... sections, or we should bring those existing one the newly introduced section.

[acolombier]

LGTM, thank you!

I will merge the PR in the main repo, but I'd like to wait for @Serveny's PR to go in before we merge this one, so this will be a great test for it.

[ronso0]

In 2.5 VuMeter has already been replaced by vu_meter (VuMeter moved to deprecated section)
Guess we need to merge 2.5 -> 2.6 -> main.
Will try