docs: how to backport ncap to an existing golang-crossbuild version

[v1v]

Trying to explain in the existing docs how the ncap could be backported to an existing golang-crossbuild version.

#315 (comment) is the one explaining these steps tool.

[elasticmachine]

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• Start Time: 2023-09-25T14:29:42.732+0000

• Duration: 48 min 19 sec

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To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

[efd6]

Thanks