docs: running the code + creating a new problem

[BenWibking]

Describe the proposal
Add a documentation page on how to run the code, following the WarpX documentation: https://warpx.readthedocs.io/en/latest/usage/how_to_run.html

Also, add a documentation page on how to add new problem.

Describe alternatives you've considered
N/A

Additional context
It's a common question from new users on Slack.