Write docs for v1 changes, guarantees strategy forward

[oleiade]

As k6 is getting ready to release its first stable major version, and establish a new release and maintenance paradigm and set of practices for the future, we aim to document most of the changes that it will involve:

• What's new in k6 version 1
• What does it mean for k6 v1 to be a stable major version
• What changes to our release process?
• What changes to our support process?
• What to expect in the future, how often will k6 be released, etc.