Documentation should mention which fields are optional in `target.json`

[Shatur]

Describe the bug

The documentation don't mark opposites as optional:

mpfb2/docs/fileformats/target_metadata.md

Line 34 in 13530a3

- `opposites` (object) — Maps opposing directions to target names:

But it can be optional. For example, in this JSON object:

mpfb2/src/mpfb/data/targets/target.json

Lines 1844 to 1851 in 13530a3

	{
	"has_left_and_right": false,
	"label": "head-triangular",
	"name": "head-triangular",
	"targets": [
	"head-triangular"
	]
	},

Same issue with unsorted. Looks like it's always either always empty or missing.

[joepal1976]

Thanks, will look at it

[joepal1976]

While checking this, I also encountered an unfortunate bug where some particular opposites are not working as intended...

Thanks for reading through the stuff and pointing out these oddities.