Include docs and tools for integrating with TileMaps

A few folks have scripted their own integration between SG Physics 2D and the TileMap node, and so it probably makes sense to include some official docs - and possibly tools - to make this easier.

At minimum we could put a script snippet in the docs that folks could start from.

However, pretty much any approach is going to involve creating SGStaticBody2D's for each tile that needs collision, which could end up being a ton of nodes. Assuming the broadphase cell size is well tuned, the physics engine should be able to performantly handle loads (ex. thousands!) of bodies and shapes, but having loads of Nodes in the scene tree can be a problem. So, I think it may make sense to add a way to create bodies/shapes in the physics engine without creating nodes (like you can do with PhysicsServer2D with Godot's builtin physics enigne) which could be used in this situation.

I don't know if it makes sense to add some kind of SGTileMap node - I worry that it'll either be a ton of work (re-implementing TileMap's features) or confusingly incomplete (where some things are safe to do, and others aren't). If it's script snippet, I think it's more OK being incomplete, because folks will know it's a hack. :-)