💥 'install' should install 'stable' instead of 'nightly' for operators/demos/etc...

[fhennig]

Acceptance criterea

• Documentation updated where the behaviour changes
• operator install installs stable by default
• demo install install stable by default

[NickLarsenNZ]

As should demo install.

[sbernauer]

Just to add: demo install installs the version the demo is build against. This could be every possible release (including dev).
That's what this field is for