Update slicing/rendering docs (napari#424)

# References and relevant issues
Closes napari#237 
Closes napari#400 

# Description
This updates the current rendering docs in napari to reflect a few
things.

1. Describe how slicing and rendering works at a very high level in
napari.
2. Remove explanations of async rendering implementations that are no
longer available on `main`.
3. Add explanations and links to NAPs/issues/PRs to implementations that
replace (2).

This is still very much a work-in-progress, so isn't worth a review yet.
But wanted to share out progress as I make it (slowly...).

Some of the old explanations include some nice context and background
material. At the time of writing, I've deleted some of these, but I am
looking for ways to integrate them back into these docs.

---------

Co-authored-by: Draga Doncila Pop <[email protected]>

Author: andy-sweet

docs/_toc.yml

@@ -65,7 +65,6 @@ subtrees:
           - file: guides/event_loop
           - file: guides/threading
           - file: guides/3D_interactivity
-          - file: guides/rendering-explanation
           - file: guides/rendering
           - file: guides/performance
           - file: guides/preferences