Documentation for envelopes for sentry-cli

[hubertdeng123]

We have documentation that covers envelopes in general.
https://develop.sentry.dev/sdk/envelopes/

However, we don't have a section that says this is available for the sentry-cli which would be nice to have. Also, there was some confusion around event_id's in envelopes and how that related to events in Sentry. As far as I can tell, it seems to be for fingerprinting in order to group events together into issues. This might be nice to document too.

[marandaneto]

Worth adding to https://docs.sentry.io/product/cli/