[8.15] Document format for air-gapped downloadable packages (backport #1305)

[mergify]

This updates section 1b) of Host your own artifact registry for binary downloads to expand on the package URL format.

it's not perfect but I don't think we have a way of listing all possible package URLs, so at least providing the format and guiding people to refer to the Elastic Agent download page can help clear things up.

Closes: #287

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This is an automatic backport of pull request #1305 done by [Mergify](https://mergify.com).

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