Move tutorial ZIPs to a dedicated repository

[YuriSizov]

Supersedes #5374, #6187, #6856. A new PR needs to be made against the new repository.

Let me introduce you to https://github.com/godotengine/godot-docs-project-starters, a new dedicated repo where we can keep and keep track of our template projects and asset sets used by tutorials. Having these projects and assets in ZIP files makes it a pain for reviewers and for bookkeeping/history preservation. So a dedicated repository where these files are tracked normally and then packed as a publishing step is the proposed solution here.

The repo is already up, it has a branch for Godot 4 and a branch for Godot 3, and corresponding rolling releases that we can link with persistent links. This PR removes the files from the documentation repository and puts in these new links instead. Here're a few notes about the changes:

• Contributing guidelines have been updated for both files and images, since that was missing some relevant information.
• pong_assets.zip seems to be unused, and therefore removed completely.
• The background loading tutorial has a single-file example, resource_queue.gd. I kept it in this repo for now, but if it ever gets expanded into a proper test project, it should also be moved.
• Some example projects are complete (-ish) and should probably be replaced with demos (e.g. the 2D movement one, 2D movement project can be replaced with a demo godot-docs-project-starters#2). But for now I simply moved them to their new home.
• Extra files, such as system files and .import folder, have been removed from example projects.
• The singleton autoload project is licensed under Apache for some reason, at least according to a file included in the ZIP. This should probably be addressed, Singleton autoload project licensed under Apache godot-docs-project-starters#1.

The contents of the new repo, in both master and 3.x branches, corresponds to the state of these ZIPs in the master branch of this repo. master branch of the new repo needs to be converted to Godot 4 syntax, and 3.x branch needs to be updated to reflect the state of the Godot 3 documentation (perhaps it's already fine as is).

[skyace65]

Looks good!

[akien-mga]

Looks great!

[akien-mga]

Thanks!