Better guides for running LMMS on Linux and/or macOS

[PhysSong]

@tresf and I talked on Discord about this.

• We have a guide for setting permissions to AppImages, but it's stable-only(see Add Linux downloads #254 for details)
• There are no notes about Mac's GateKeeper, e.g. https://support.apple.com/kb/ph25088?locale=en_US

Proper notes will help users troubleshooting problems when trying to execute LMMS.

[tresf]

@tresf and I talked on Discord about this.

• We have a guide for setting permissions to AppImages, but it's stable-only(see Add Linux downloads #254 for details)
• There are no notes about Mac's GateKeeper, e.g. https://support.apple.com/kb/ph25088?locale=en_US

Proper notes will help users troubleshooting problems when trying to execute LMMS.

Now that Apple no longer offers a Right Click -> Open workaround, the gatekeeper steps are much more grave, recommending to move lmms.dmg to the recycling bin. The new steps are:

sudo xattr -d -r com.apple.quarantine /Applications/LMMS.app

A better short-term solution is to just buy an Apple Developer ID and sign / notarize these installers:

• App is no longer signed (10.14 build, running on Ventura) lmms#7070
• MacOS Signed Installer lmms#3601

A long-term solution is to establish an organization for LMMS that can have the certificate issued to. This is explained in-part here:

• Windows Signed Installer lmms#3602