Added custom node rendering example

[Lindenk]

Objective

Provide the fruits of my keyboard mashing as an accessible example for creating a custom rendering node.

Solution

Provides a well commented and (hopefully) idiomatic example for creating a rendering node and adding it to a node graph.

[DJMcNab]

I think it would be useful to have a screenshot/video of what this example produces here.

[Lindenk]

Here you are. I went with something that uses the resolution from ExtractedView, a simple implementation for extracting Time, and the color is extracted from a custom node attached to the camera. I feel like that should provide decent coverage for what a user might want to do

[Lindenk]

I'm also a little unsure how to fix the check_bans check. It seems to pass on my end

[djeedai]

My 2c is that it's a unique occasion to explain what a render world is, the extraction process, etc. All these things that should be documented but are not. Instead, I don't really see the difference between this example and reading the internal code for e.g. sprite or UI rendering. So I'd suggest to add a lot more comment not only on what is going on but first and foremost why, and how the general process of adding a render node works.

[Nilirad]

This PR adds a new example. Adding module and item level doc comments, as described in:

• Module-style doc strings for examples #3951 and
• [Merged by Bors] - Doc/module style doc blocks for examples #4438,

would be really useful to those who will browse examples.

[IceSentry]

Most of this is now covered by the post_processing example.

A more targeted example like this might still be useful but this PR is now very outdated to the point I believe it should just be started from scratch.