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.
Since we don't want to come up with a GUI maintenance system and
instead focus on the steering behaviors, all relevant variables have
been moved as exported in the main demo script for each toy.
Changed text boxes to sliders to improve usability. This also involved
adding some simple graphics, since the _draw method is not
anti-aliased and doesn't look great.
The use of multiline blocks in GDSCript for comments leaves them in the
final compiled file, increasing size and potentially processing for no
end-user gain.
