Documentation fixes for new users

[Alan-Goodman]

I would suggest changing the example given here
https://desc-docs.readthedocs.io/en/stable/adding_objectives.html
to be a quantity that isn't pre-calculated in DESC (i.e. have a function that penalises mirror ratio, where you actually calculate the mirror ratio, rather than just reading it from a dictionary) so that the user learns to write their own targets.

I would also suggest making an example script where a GenericObjective and an ObjectiveFromUser are optimised in a loop. These are probably the most important things for a new user to learn to use, and having a specific example would be useful. One example would be optimising for QH with:

• 0.81 < iota_ax < 0.82, and
• 0.96 < iota_edge < 0.98

This iota target function would be written from-scratch as an ObjectiveFromUser, and would be defined like:
def IotaTarget( ..., iota_ax_target=0.8, iota_edge_target=0.97, iota_ax_tolerance=0.01, iota_edge_tolerance=0.01):

I think these would be super helpful for new users to learn!

Cheers,
Alan

[f0uriest]

While we're at it there are also some recently added objectives that aren't in the docs (ballooning, power balance, some of the new FixNearAxis*)