Update documentation on custom units

[Brunni]

The documentation here is outdated. Some of the method parameters are deprecated:
https://github.com/angularsen/UnitsNet/wiki/Extending-with-Custom-Units

Also it is not really clear, if it makes more sense to implement IQuantity instead of IQuantity.

Would it be possible to use the generator also for custom units?

[angularsen]

Yes that part of the wiki needs some love. Extensibility is still a bit work in progress, and the least mature part of the library. Generally we are going away from enums and towards string based values in v5 (#982 ), which helps a bit.

I believe the wiki is editable by anyone, so please just add your suggested changes to it.

if it makes more sense to implement IQuantity instead of IQuantity.

Typo here maybe? IQuantity instead of..?

The generator is currently not easy to reuse for custom units. The current design allows adding units, conversions, abbreviations etc at runtime, but not at compile time. The new source generators might enable something in that direction, but I haven't digested the possibilities in that area yet: #902