feat: Light and dark mode repo banner updates

[danielgolden]

Description

• Updates the existing category images to use the new NR open source logo
• Updates the existing category images to use the new New Relic logo
• Added dark mode versions of each category images (in a /dark directory)

These were mocked up in a Figma file which anyone at New Relic with Figma access has access to.

Preview

The category image below will be displayed in light or dark mode depending on your GitHub settings.

Here's how that preview works

<picture>
  <source 
    media="(prefers-color-scheme: dark)" 
    srcset="https://user-images.githubusercontent.com/812989/179037509-85aa1cf2-ed1d-4e6c-9f76-57b228387ab4.png">
  <img 
    alt="A New Relic Open Source community project banner. It highlights that this repo is an official New Relic open source community project, supported by a maintainer(s) within New Relic." 
    src="https://user-images.githubusercontent.com/812989/179037462-418a4f19-da87-4adf-af8e-657bbd909d5b.png">
</picture>

[jbeveland27]

Note: there's 2 follow-up items:

• Correct the copy snippets we made available from the opensource website (this has a link in the sidebar pointing to the wiki).
• Update the repolinter rulesets with the new copy snippets