* 📝[#2884] Documentation Service Fetch

* 🩹[#2884] Fix Service fetch action not using var key.

This would break when names had upper case or diacritics.

* 🩹[#2884] Set the default body of ServiceFetchConfig to null

Quality of life patch; some services —like dummyJSON.com— 400 or 500 on
an unexpected body on a GET request.

Current workaround is select POST change body to null then select GET
again.

* 📝[#2884] Add warning for experimental nature.

- warn against common pitfall
- point towards issue backlog and warn those are not part of a current
  roadmap/sprint
- document current state and nature of variable interpollation

* 👌[#2884] Process review comments

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Co-authored-by: Chris Wesseling <chris@maykinmedia.nl>

Backport-of: open-formulieren/open-forms#2906