docs: add guide

[uncenter]

Attempt at writing down how Whiskers works. Probably not how we will approach this but I wanted to at least get the ball rolling.

[sgoudham]

I think one thing that will help break the wall of text is the inclusion of visual mermaid diagrams to demonstrate how a simple matrix command might work or the pipeline of steps that whiskers performs in order to go from template -> rendered output. And/or also maybe some nicely rendered ascii trees.