💥 '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

See also: #389

[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

[sbernauer]

Maybe unpopular opinion: I prefer to leave "operator install" at the nightly

[NickLarsenNZ]

Maybe unpopular opinion: I prefer to leave "operator install" at the nightly

I disagree. It should do stable stuff by default. Developers/users needing nightly should be opting in.