Use consistent best-practices in UI examples

[TimJentzsch]

Background

I'm building a UI-heavy app after not having worked with bevy_ui for quite some time.
I now want to learn common patterns and best practices on how to build a UI with Bevy.

To my knowledge, there is no higher-level documentation available, so I turn to the examples/ui folder.
However, the patterns used there seem to be quite inconsistent, so I'm not sure what I should use when.

For example, compare grid.rs and image_node.rs.
The grid example makes heavy use of the .with_children syntax and uses shorthand functions like px and percent.
The image example instead uses the children! macro and the Val::Px constructor.
Then the standard_widgets.rs example makes heavy use of the impl Bundle pattern, combined with children!, but also uses Children::spawn.

So I think after looking at these examples, I'm more confused than before.

How can Bevy's documentation be improved?

I'd like us to:

1. Define (somewhere) some guidelines, on when to use which pattern (or maybe which patterns shouldn't be used at all anymore?)
2. Consistently apply these guidelines to the UI examples
3. Ideally also add more inline comments to better explain these things in the examples. There we need to find some balance between teaching important concepts and not repeating the same stuff in every example.

[ickshonpe]

I'd suggest splitting this up into some sub-issues:

1. Use consistant spawning patterns in the UI examples

   The inconsistant spawning patterns are just because a lot of the examples were written back when there weren't any alternatives to with_children. There's little point atm in doing this until the release of bsn though, so it should probably be labelled blocked.

2. Use the shorthand functions to construct Vals and UiRects in the UI examples

   IMO the shorthand px, percent, vw, etc constructor functions should be strongly preferred.

3. Remove redundant layout constraints from the UI examples

4. Fix inconsistent and non-idiomatic CSS constraint usage