0.15 Migration Guide: Provide examples for creating `Rect` with logical values (missing `logical_rect` method on `Node` - also see PR 15163 and PR 16375)

## Problem

From 0.14 -> 015 there are two changes that have led to my (toy) project being broken and I couldn't figure out for a while why it broke (⚠ I'm very new to GameDev and Bevy).

- the first change, introduced in [PR 16375](https://github.com/bevyengine/bevy/pull/16375), states that `ComputedNode`’s fields and methods now use __physical coordinates__, instead of logical coordinates and by multiplying the physical coordinates by the `inverse_scale_factor` will give the logical values.
- the second change introduced in [PR 15163](https://github.com/bevyengine/bevy/pull/15163), states that methods `logical_rect` and `physical_rect` have been removed from `Node`

IMHO, we should __provide examples__ on how to reimplement `logical_rect` for oneself, if one decides that it is a useful method to have (I've done this now after migrating from 0.12. -> 0.15 by implementing an extension trait on `Node`).

Also both changes mentioned above should reference each other, because one change alone would probably have a small breaking impact, but in combination they have high potential for "invisibly" breaking apps (aka no compile errors, but wrong logic).

## Proposed solution

An example that we should provide in [migration guide for PR 15163](https://github.com/bevyengine/bevy-website/blob/5115c66f937a306996de3b4fb9c53c03e7219e76/release-content/0.15/migration-guides/15163_use_precomputed_border_values.md?plain=1), while referencing the [other changes regarding `ComputedNode`](https://github.com/bevyengine/bevy-website/blob/5115c66f937a306996de3b4fb9c53c03e7219e76/release-content/0.15/migration-guides/16375_Only_use_physical_coords_internally_in_bevy_ui.md?plain=1#L2):

0.14:
```rust
fn my_system(q_nodes: Query<&Node, &GlobalTransform>) {
for (node, g_trans) in &q_nodes {
let logical_rect = node.logical_rect(g_trans);
}
}
```
0.15:
```rust
fn my_system(q_nodes: Query<&Node, &ComputedNode, &GlobalTransform>) {
for (node, comp_node, g_trans) in &q_nodes {
let logical_rect = Rect::from_center_size(
// we convert both values to their logical values
g_trans.translation.xy() * comp_node.inverse_scale_factor(),
comp_node.size() * comp_node.inverse_scale_factor());
}
}
```

Start with release-content/0.15/migration-guides/15163_use_precomputed_border_values.md and review the linked 16375_Only_use_physical_coords_internally_in_bevy_ui.md guide. Add the requested 0.14 and 0.15 Rect examples and cross-reference the two migration changes. Done means the guide explains how to recreate logical values after the Node methods were removed.

Written by the indexing model from the issue text.