Content: Define operand concept, simplify graph connection steps

As discussed in webmachinelearning#572:

- Define an "operator" concept, with inputs, outputs, activations.
- Defer "platform operator" and "platform operand" to build.
- Standardize the graph connection steps across builder methods.
- Simplify activation, input and constant steps.

Not covered in this change:

- Erroring if input's [[builder]] doesn't match `this`
- Build algorithm - covered by webmachinelearning#448 and webmachinelearning#457
- gru() is missing steps to populate output. Added "Issue" note.
- Introducing "Validate arguments" section for each method.
- Introducing "Calculate output shape" section for each method.

For webmachinelearning#549 and webmachinelearning#572.