docs: diff 7 and 8 (#2561)

### Description

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Detailed diff between <7 and 8, may be useful for issues such as #2552
and others.

### QC
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* [ ] The PR contains a test case for the changes or the changes are
already covered by an existing test case.
* [ ] The documentation (`docs/`) is updated to reflect the changes or
this is not necessary (e.g. if the change does neither modify the
language nor the behavior or functionalities of Snakemake).