Add quickstart guide and quickstart demo
Authored by Razoric and edited by Nathan and Johnny.
The purpose of the guide is simply to give an overview of what the
behaviors are there for, and a quick look at how to actually make use
of them in the basis of this toolkit, with some explanatory comments.
The actual user's manual will be built out of the API reference.
The use of `not is_inside_tree()` before setting the setting class' value
meant that the starting values would always be the default. Moving the
value setting before checking for tree readiness fixes the issue.
Used `var variable := 0.0` as discussed in the Godot issue, instead of
`var variable: = 0.0`.
Mostly these are minor/cosmetic changes, but I've also reorganized the
folder structure (naming of folders) to reflect our guidelines, plus
made some minor changes to the demo codes. Still work in progress.
