Update dark theme handbook with suggestions for swapping images between light/dark mode

[mcoker]

https://github.com/patternfly/patternfly-org/blob/main/packages/documentation-site/patternfly-docs/content/developer-resources/dark-theme-handbook.md

Towards https://github.com/patternfly/patternfly/issues/5613, we should reference a few ways of managing displaying images/logos (though this could apply to other things as well) between dark/light theme. There is a core issue here to come up with some recommendations we can support - https://github.com/patternfly/patternfly/issues/6362

Some thoughts:

• Use inline SVG's with fill colors set to PF CSS vars that adapt to light/dark theme

• Use colors that work well with light and dark theme (penta colors and PF chart colors have palettes that work with light and dark)

• Utilize background color, border color, border-radius, and padding/spacing to create a container that can hold dynamic/user-supplied images (hinted at in this comment)

• Write CSS (manually or as a common/utility class)

  <img src="/light-theme.jpg" class="show-in-light">
  <img src="/dark-theme.jpg" class="show-in-dark">
  
  .show-in-dark, 
  .pf-v5-theme-dark .show-in-light {
    display: none;
  }
  
  .pf-v5-theme-dark .show-in-dark {
    display: revert;
  }