Add tutorial on computing chi²

[Zaharid]

The way we use and define objective functions and chi² related metrics has many knobs, which makes it difficult to use and interpret correctly without a lot of context. It would be good to have a tutorial that lists the possible options, their applications and the relevant vp flags.

These are at least:

• t0
• use_pdferror
• regularization
• weights

[RoyStegeman]

Pretty sure at least some of these are defined in the docs where they are relevant. So just to be clear, is your idea to make a separate page collecting all of them in a single page and just have the info in the docs twice, and/or cross referencing the places where they are already discussed?

I agree that, now that there are external users, such a page might be useful.

[Zaharid]

Yes indeed. It seems clear that having a page that can be referenced in one go would be useful.