Add tonemapping example images for use in the book and rustdoc

[superdump]

No description provided.

[doup]

If these are going to be referenced from the API reference (not the website docs) then I think it's better to move it into a separate assets-docs folder so it's not removed in case there is a website redesign (or tooling is used to optimize the website assets, etc.).

If this approach is used, then I would also add a assets-docs/README.md to explain the purpose of this folder.

[alice-i-cecile]

Seconding doup's suggestion. Once that's done this LGTM.

[superdump]

Done.

[mockersf]

Do we have an idea on how to handle versioning if we notice a bug in how those images were generated and need to be updated, but still have to link to a before fix version in the corresponding docs?
I think putting the version of Bevy used to generate them either in a folder or in the filename could be a start.

[doup]

Maybe just version the files like foo.v0.webp? Without adding a relation to any Bevy version.

[mockersf]

rust-lang/rfcs#3397
It also mentions a few alternatives that would work today

[Selene-Amanita]

A bit late to the party:

This image should probably be moved there too: https://github.com/bevyengine/bevy/tree/main/assets/docs ?

(Created in bevyengine/bevy#9061 )