docs(config): document that groups compose via flows, not nesting

[quike]

Closes #37.

Item 3 of #37 asked for a decision, not code: should commands: entries be able to call another group, the way Taskfile allows {task: other-task}?

Decision: won't do. This documents it so the question does not come back as an undocumented gap.

The reasoning, as laid out in #37: a group is keepup's atomic unit — one cache fingerprint, one require:/skip-if: gate, one node in the graph. Nesting would have to answer whose cache: applies, whose gating runs, and what a cache hit on the inner group means for the outer one, while re-creating the ordering steps:/run: already express.

Adds a Groups do not nest section to docs/CONFIG.md stating the rule, the reasoning, and the idiom to use instead — extract the shared work into its own group and name it from both flows — with a worked example. It ends by inviting a reopen if someone has a case that neither commands: nor a flow can express, since a documented decision should not be a closed door.

Docs only; no behaviour change.

With items 1 (#52) and 2 (#53) already merged, this retires #37.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.97%. Comparing base (d8653eb) to head (2e393cd).

Additional details and impacted files

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  Coverage   92.97%   92.97%           
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  Files          27       27           
  Lines        1565     1565           
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  Hits         1455     1455           
  Misses        107      107           
  Partials        3        3           

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[quike]

🎉 This PR is included in version 1.31.1 🎉

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