Clarify extension ID creation

[andrewbaxter]

Description

For manifest V3, the developer must come up with an ID on their own to submit the extension. I clarified it (I think in the best location) and provided details on the requirements and how to to choose one.

Motivation

AFAICT nowhere in the docs did it say that the developer can come up with the extension ID on their own, and in several places it's suggested that they don't come up with it on their own (ex: testing locally, uploading manifest V2 extensions with no ID).

This confused me for several hours until I joined the addons web chat and got an answer from someone official.

Additional details

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