Lacking Documentation on Deferred & Forward Rendering

## How can Bevy's documentation be improved?

Documentation regarding the Forward/Deferred rendering methods is spotty, and is all-around missing important information. The actual implementation is barely touched on from what I can find.

First off, I think improvement is necessary for the description in the [Deferred Rendering Example](https://bevy.org/examples/3d-rendering/deferred-rendering/), as it's the first, and most clear result in searches.

Second, we really need more concise, and less spotty info of the implementation. It's nice to get some info that touch on the internals. Mostly, we need a list of built-in rendering features available with each method (Forward, Deferred, & Forward + Prepass).
Ideally, we'd have some list, such as a 3-column table (though this might be overkill), detailing feature availability on each mode. An example being the anti-aliasing methods (such as MSAA unavailable on Deferred).

Additionally, the existing page for [DefaultOpaqueRendererMethod](https://docs.rs/bevy/latest/bevy/pbr/struct.DefaultOpaqueRendererMethod.html) needs documentation. The documentation necessary here would be minimal, explaining basic behavior and linking to the underlying [OpaqueRenderMethod](https://docs.rs/bevy/latest/bevy/material/enum.OpaqueRendererMethod.html).

The most important details here appear to require more internal knowledge on the pipeline.

Start with the Deferred Rendering Example and the DefaultOpaqueRendererMethod and OpaqueRendererMethod documentation linked in the issue, then read the rendering pipeline implementation to verify feature differences. Done means the example is clearer, the Forward/Deferred/Forward + Prepass feature availability is documented, and DefaultOpaqueRendererMethod explains its behavior and links to OpaqueRendererMethod.

Written by the indexing model from the issue text.