Documentation/instructions for repeating data challenge

Having tried to follow the instructions in the README, I think they could be more specific about the steps needed to repeat the data challenge exercise, at least to some approximation (i.e. repeating it but not necessarily reproducing it exactly).

Specifically:

__surveysim__

* Can the surveysim files from the data challenge be copied directly to skip this step or do they need to be regenerated?
* In the line `surveySim((2020,1,1), (2020,6,1), tilesubset=tilelist)`, which start and end dates should be used? Why are these dates given in the example -- they don't look like the full survey.

__mock target catalog__

* What is the purpose of the option `--realtargets /project/projectdirs/desi/target/catalogs/targets-dr3.1-0.8.1.fits`?

__quicksurvey__

* Two alternative ways to run quicksurvey are given. Which is the default, and what is the purpose of the other way?

* The two ways are:
1. Specify `--obsconditions` and `--fiberassign_dates`
2. Specify `--epochs_dir` and `--n_epochs`

Are these two combinations mutually exclusive?

* Where does `fiberassign_dates.txt` come from?

__Bright time only__

I'm only interested in repeating the bright time part of the survey. It's not clear if/how this can be done independently (i.e. without spending time on the dark/grey epochs and without including those mocks). That could also be mentioned in the instructions. I understand how to exclude the mocks from the `input.yaml` files for `select_mock_targets`, but not if/how this is supported by `surveysim` and `quicksurvey`.

Start by reading the README's data challenge instructions and trace the surveysim, mock target catalog, quicksurvey, and select_mock_targets steps. Check the stated options, dates, fiberassign_dates.txt, epochs, and input.yaml references; done means the README explains how to repeat the exercise and run the bright-time portion independently, addressing the listed questions.

Written by the indexing model from the issue text.