Revisit workflow of Qiskit Serverless docs

[abbycross]

Closes #4144. cc: @pandasa123 @acastellane @ElePT

• Include a high-level tl;dr: overview of the steps
• remind users their serverless workloads won't show up on their Workloads table
• be clear about which saved account is default (this applies not just to serverless)
• where to go to find out about package versions/dependencies (also a larger question)
• add info about when a job id is actually assigned
• clearer connection with Functions?

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[abbycross]

When @pandasa123 is back, need his feedback on the admonition regarding link-outs to the github.io page:

• Language ok?
• How many Serverless pages do we post the admonition on (just the intro, all of them, etc.)?

[abbycross]

Possible to state in our docs when the job_id is assigned in the process? - or will that be changing too? @ElePT