Add metadata design doc to table of contents

[xylar]

Also, a little clean up.

Checklist

• Documentation:
  • Design document has been generated and added to the docs
  • Documentation has been built locally and changes look as expected

[xylar]

@philipwjones, in the process of looking at #19 (including building the docs locally), I noticed this design doc missing from the table of contents. I overlooked this in my review of #13.

[philipwjones]

Thanks for catching - both the internal formatting and missing index entry. Looks fine by inspection.

[xylar]

Okay, feel free to merge whenever you're ready. I don't have integrator right.