Adding context menu example

[extrawurst]

Objective

Provides usage example of a context menu

Testing

• Tested on MacOS
• Tested on wasm using bevy_cli

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Showcase

Screenflick.Movie.120.mp4

[github-actions]

You added a new example but didn't add metadata for it. Please update the root Cargo.toml file.

[Vrixyz]

• I'm approving if the CI goes green ; this PR is conflicting with New cooldown example #19234 and the other PRs adding the new usage folder.

[extrawurst]

@Vrixyz added a bunch more docs and cleanups. happy to hear what you think

[Vrixyz]

I like this example, other than a small nit on docs which may not be blocking, I added a comment on there which may be important to make sure we understand what this example does: https://github.com/bevyengine/bevy/pull/19245/files/eda7098b2b4e9a281253705b458cab263420fc57#r2094492441

[NiklasEi]

Some small comments, but I like this example!

[BenjaminBrienen]

I think it would be good to have the context menu only appear when Mouse2 (or secondary pointer input button) is pressed, as that is more common.

[extrawurst]

@BenjaminBrienen I would love to not do that for two reasons:

1. it will make the example not work on mobile browsers (would like this to be working out of the box on the web as well when folks check it out on phones)
2. it adds complexity (not much of course but nothing that cant be found in other examples)

[BenjaminBrienen]

I don't think floating context menus are the appropriate way of getting input from touchscreen users, but I think your points are very valid.

[alice-i-cecile]

This is probably going to get merge conflicts, but I'll try merging and see if anything breaks <3

[BenjaminBrienen]

Has merge conflicts 😋

[extrawurst]

@BenjaminBrienen Ahh the new BorderColor's - should be fixed now