Document the configuration system of each component

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Migrated from T1388 (view on Phabricator)

added Documentation priority:Normal labels

I've attached here T917, which was just a specific instance of this.

changed title from Document the configuration system for each piece of softare of the SWH infrastructure to Document the configuration system of each component

See also swh-core#826 (closed) which is somewhat related

@douardda Can you please tell me more on this issue . I want tot work on it

marked this issue as related to swh/meta#4082

mentioned in issue swh-core#1410

mentioned in issue #1758