docs: Describe metadata formats more precisely, and mention github and gitlab's

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Migrated from D7801 (view on Phabricator)

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Patch application report for D7801 (id=28180)

Rebasing onto 27d3c8a6...

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commit cb12394c34cfc691244dbc0cc5b07cf72941106e
Author: Valentin Lorentz <vlorentz@softwareheritage.org>
Date:   Tue May 10 14:05:02 2022 +0200

    docs: Describe metadata formats more precisely, and mention github and gitlab's

See https://jenkins.softwareheritage.org/job/DSTO/job/tests-on-diff/1610/ for more details.

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