Migration guide: optional outputs

[MetRonnie]

A page in the detailed description of major changes that gives an example of a Cylc7 branched graph that works in back-compat mode but fails once suite.rc has been renamed to flow.cylc, and how to upgrade to optional outputs

[oliver-sanders]

Due to the validation of optional outputs, almost all branched workflows will fail Cylc 8 validation (post-upgrade) until upgraded to use optional outputs (because Cylc knows that :success and :fail are mutually exclusive).

There is a worked example in the suicide trigger section which shows how to upgrade from suicide triggers to optional outputs.

That page links to the graph branching section which outlines common patterns with optional outputs.