Create and use a unified example color palette

## What problem does this solve or what need does it fill?

The examples currently use a huge array of colors, for no good reason.

This is hard to modify, distracting and frequently ugly. This problem is particularly bad with text, with everything from fully saturated RED to BLACK to WHITE to 90% grey, 80% grey, GOLD and arbitrarily selected SRGB values being selected.

## What solution would you like?

Add a simple `bevy` color palette to `bevy_colors`. Include our default clear color in there, and otherwise steal branding colors from the website. Look through the examples and slowly replace all other color palettes.

## What alternative(s) have you considered?

Use the `tailwind` color palette everywhere. It won't be as coherent (since it's very large) or easy to change (since it has an external definition), but at least it won't be as 80s.

## Additional context

Encountered during #12163. This work cannot be finalized until that PR is merged.

Start by reading the bevy_colors crate and reviewing the examples' existing color palettes, then check the website branding colors and the default clear color. Done means a simple Bevy palette exists in bevy_colors, includes the default clear color, and the examples use it consistently; this work depends on #12163 being merged.

Written by the indexing model from the issue text.