Bundle example

[alice-i-cecile]

What problem does this solve or what need does it fill?

Bundles are an important piece of the core ECS design, but aren't covered very well by the existing examples.

What solution would you like?

An example showing how to create bundles is made. This should include more advanced patterns, like merging bundles, creating a trait for a specific class of bundles etc.

What alternative(s) have you considered?

This could instead be added to ecs_guide, or more game examples. I'm not super keen on this, since it makes it even harder to search for.

The docs.rs documentation could also be improved, which is probably worth doing at the same time regardless.

Additional context

Once #792 is solved, an explanation for how to merge bundles ergonomically should be added to this example.

If spawn_batch still exists, showing how to use these bundles with spawn_batch is very valuable.

[alice-i-cecile]

Resolved by #1570, which added docs to the Bundle trait directly.