Update documentation for how to use the new update script

[foolip]

What would you like to see added to BCD?

#19971 moved the update-bcd script from mdn-bcd-collector into BCD, but we have not updated documentation for how to use this new script. https://github.com/mdn/browser-compat-data/blob/main/docs/contributing.md#optional-generating-data-using-the-mdn-bcd-collector-project links to https://mdn-bcd-collector.gooborg.com/docs/update-bcd.md.

How impactful do you think this enhancement will be?

The main thing is that we should avoid any divergence between the original update-bcd and the one that was moved into BCD. Although a bit painful at first, I think we should do this by ensuring that the script in BCD itself is fully up to the task, and refer to it in docs.

Do you have anything more you want to share?

cc @Elchi3 @queengooborg

[Elchi3]

Closing this as we want to keep the update scripts with the collector. I will take a look at the docs if they are good.