Add 3D Printable Rack Options Section

I think it could be beneficial to add a section to the main page of this repo for user-created model files for various rack parts. Instead of purchasing a 1U SBC shelf and putting your SBC on it, maybe someone has created a 2U front plate with mounts for the SBC you're using. I think it would be neat to see that collection of parts grow, as new technology is released and people create custom methods of incorporating it into a mini-rack. Just a thought. And thanks for everything, @geerlingguy

Start by locating the repository's main page and reviewing how existing project resources are organized. Add a section for user-created 3D model files for rack parts, with enough context for users to find and understand the collection. Done means the section is visible on the main page and clearly presents these community-created options.

Written by the indexing model from the issue text.