Document zebrad download command

[oxarbitrage]

Zebra has a command to pre download the parameters. Zebra also download parameters at zebra start if they are not present.

There is no documentation about this, i think we should add:

• Briefly describe what are this parameters and why they are needed.
• Document the zebrad download existence.
• Explain zebrad start will automatically start downloading parameters at startup.

https://zebra.zfnd.org/user/run.html

We may need to wait on #3133 as some of the current behaviour will maybe change.

[teor2345]

This isn't a breaking change, does it need to happen before the stable release?

[conradoplg]

This isn't a breaking change, does it need to happen before the stable release?

@teor2345 I moved some to that Epic thinking about making it just a "Stable Release" epic, but there weren't enough issues to justify that. I'll move them to the Tech Debt Epic

[mpguerra]

@oxarbitrage can you please add an estimate to this issue since you worked on it?

[mpguerra]

I'm not going to move this one to Sprint 2 as it was just supposed to be work for the end of year. We should try to get its PR reviewed and merged soon enough though.