Write parity conservation example notebook

[redeboer]

• Explain mechanism (LaTeX and markdown only)
• Work out example for 𝐽/𝜓 → 𝐾⁰𝛴⁺𝑝̅
• Illustrate with a code example

[redeboer]

@Leongrim just add the notebook under docs/usage (best to copy one of the other notebooks there to have the same format).

If you want to build the notebook with HTML, add its name here

qrules/docs/usage.ipynb

Lines 251 to 255 in 1f38b20

	"usage/reaction\n",
	"usage/particle\n",
	"usage/visualize\n",
	"usage/conservation\n",
	"usage/custom-topology\n",

and run tox -e doc (or tox -e docnb to generate cell outputs). More info on building documentation here.

[Leongrim]

I'm not sure if I did everything correctly.