Add low level post process example using a custom render pass

[IceSentry]

Objective

• The current post process example is very high level and inefficient.
• A lot of people want to dive deeper in bevy's rendering, but we don't have any low level rendering examples and the source code is very sparse in terms of docs explaining what's going on.

Solution

• Use the post process features added in 0.9 in an example with a bit more docs.

Notes

The actual shader is pretty much the same as the current post_process example but adapted for the custom render pass. So it's still just chromatic aberration. This could probably be something slightly more complicated to show using more bindings. For example the offset could be configured from a binding instead of being hardcoded.

I originally wanted to use this example to also teach about all the various render stages, but it's already quite complex so I chose to keep it simple.

[JMS55]

Lots of minor suggestions on what to reword and explain better.

Major stuff I noticed missing:

• No usage of prepare/queue systems to create textures or bind groups
• No usage of specialized pipelines and per-view pipelines, or shader defs for when runtime uniform settings aren't desired
• I feel like we should mention that you you can use fancier filters and stuff with ExtractComponent, implement it manually instead of deriving, or implement it entirely manually with a system. Maybe this should go into the rustdoc, though.

Random other thoughts:

• Node boilerplate is pretty bad, can't wait for you to improve it
• Perhaps we should take ExtractComponent/Uniform one step further, and add some kind of template trait for preparing a list of textures based on descriptors, preparing a bind group, creating one or more specialized pipelines, and then possibly integrating the pipelines into a render node. Would greatly simplify TAA/SSAO/etc.