Fixes/prunes broken nav links.

[robrussell]

I ran MkDocs locally and tracked down the broken links in the navigation. Here are the details for the individual changes:

• Changes nav link from https://opensourcerover.jpl.nasa.gov/ to https://jplopensourcerover.com/. The .gov site is down and it looks like https://jplopensourcerover.com/ is the replacement but tbh there's no certain way to verify this.
• Adds missing links to mechanical/body/README.md and electrical/wiring/README.md. Reorders mechanical section to reflect build order.
• Updates link from optional_cad_parts/README.md to mechanical/optional_cad_parts/README.md. File was moved in 35d8e9b. The file is out of date though so maybe this link should be removed.
• Removes links to software/README.md, mechanical/body_assembly/README.md, and electrical/calibration.md. Files were deleted in 35d8e9b. Calibration is now covered in step 6 of electrical/pcb/README.md.
• Removes link to mechanical/head_assembly/README.md. File was deleted in c0c87c3.
• Removes link to mechanical/differential_pivot/README.md. File was deleted in d092d83.
• Removes links to mechanical/corner_steering/README.md and mechanical/mechanical_integration/README.md. Files were deleted in 6113a69.
• Removes link to electrical/electrical_build.md. File was deleted in 7037209.

There are still some warnings from MkDoc that aren't addressed:

$ mkdocs serve
INFO     -  Building documentation...
INFO     -  Cleaning site directory
INFO     -  The following pages exist in the docs directory, but are not included in the "nav" configuration:
              - electrical/pcb/control_board/readme.md
              - electrical/pcb/control_board/BOM/v2.0.0/README.md
              - electrical/pcb/control_board/BOM/v2.0.1/README.md
              - electrical/pcb/control_board/documentation/v2.0.1/README.md
              - examples/README.md
              - mechanical/body/laser_cut_parts/README.md
              - parts_list/extra_parts.md
              - parts_list/screws.md
WARNING  -  Documentation file 'README.md' contains a link to 'LICENSE.txt' which is not found in the documentation files.
WARNING  -  Documentation file 'electrical/pcb/README.md' contains a link to 'electrical/pcb/6.1-RoboClaw-Testing-and-Verification' which is not found in the documentation files.
WARNING  -  Documentation file 'mechanical/rocker_bogie/README.md' contains a link to 'integration/README.md' which is not found in the documentation files.
WARNING  -  Documentation file 'mechanical/wheel_assembly/README.md' contains a link to 'mechanical/electrical/wiring/README.md' which is not found in the documentation files.
WARNING  -  Documentation file 'parts_list/README.md' contains a link to 'parts_list/mechanical/body/laser_cut_parts/README.md' which is not found in the documentation files.
WARNING  -  Documentation file 'parts_list/extra_parts.md' contains a link to 'parts_list/mechanical/body/laser_cut_parts/README.md' which is not found in the documentation files.
WARNING  -  Documentation file 'parts_list/screws.md' contains a link to 'parts_list/parts_list/README.md' which is not found in the documentation files.
INFO     -  Documentation built in 0.41 seconds
INFO     -  [11:04:01] Serving on http://127.0.0.1:8000/

[Achllle]

Awesome, looks good to me!

I created #467 and #468 to track the outstanding issues you found.