improve body build documentation & images - increase nut count

[knickels]

This PR adds step-by-step instructions & images for the body box build.
I also found that I didn't have enough M4 nuts, so I added a bag to the parts list.

[Achllle]

Looks great, thank you!
What are the ODG files for? I don't see them mentioned in the docs anywhere and I'm not familliar with that format.

[knickels]

They are libreoffice drawing files used to do the image annotations. Was reading the OSHWA guide and we are supposed to: " The instructions could be in a variety of formats, like a wiki, text file, Google Doc, or PDF. Remember, though, that others might want to modify your instructions as they modify your hardware design, so it’s good to provide the original editable files for your documentation, not just output formats like PDF. "

[Achllle]

I'd rather not add them because it makes the repo heavier and the images are fairly easy to reproduce. I'm mostly worried about changing the instructions and forgetting to update those files.

[knickels]

I'd rather not add them because it makes the repo heavier and the images are fairly easy to reproduce. I'm mostly worried about changing the instructions and forgetting to update those files.

fair enough. removed.

[Achllle]

Thank you!