Add `EntityCommands::observe_self` for easier self-targeting observers about bevy

[QueenOfSquiggles]

Just for tracing the PR for the website example is here

[QueenOfSquiggles]

I'm still learning the intricacies of rust, but I wager I can at least make a simple implementation of this functionality. At the very least I'm going to give it my best ❤️

[alice-i-cecile]

Awesome, thanks! Let us know if you need any help.

[QueenOfSquiggles]

From a failure to compile on the examples, I found this chunk of code in the Observers example

(examples/ecs/observers.rs:93-98)

    commands
        .spawn(Mine::random(&mut rng))
        // Observers can watch for events targeting a specific entity.
        // This will create a new observer that runs whenever the Explode event
        // is triggered for this spawned entity.
        .observe(explode_mine);

This seems to imply the desired functionality has already been merged? Unless I'm misunderstanding, which is definitely possible.

[alice-i-cecile]

Oh! I must have misinterpreted the API: it was changing last minute. Are you able to verify and then update the example at https://github.com/bevyengine/bevy-website/blob/main/release-content/0.14/release-notes/10756_hooks_and_observers.md for me @QueenOfSquiggles?

[QueenOfSquiggles]

Sure thing @alice-i-cecile . I'll get that started in a bit here