Developer Documentation

[ashbaldry]

From @ramiromagno:
A technical vignette explaining key operations could invite contributions more easily:

Key operations:

1. Traverse graph from root to leaves
2. Evaluate leaf nodes (inputs) to concrete values
3. Deparse branch nodes (reactives) to code
4. Linearize into topologically-ordered script
   Essentially deparse() for reactive graphs, where inputs get resolved to values and reactives expanded to definitions.

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