Add docs on converting mean anomaly to true anomaly

Orbit.from_classical “only” accepts a true anomaly. It lacks support for a mean anomaly as an alternative. poliastro also lacks a method for converting from mean anomaly to true anomaly in its angle module.

💡 Feature request

A good, working and MIT-licenced implementation for converting from mean to true anomaly can be found here. It could be adopted and accelerated with numba.

🎯 Goal

Virtually every database for solar system objects’ orbital elements provides mean anomalies.

[astrojuanlu]

(Just testing the new equations feature 😍)

[s-m-e]

What about “In depth”?

Informal way of saying “discussion”, is not it 😉 ? Sounds good.