Make GfRC retargetable

CC @hasufell @simonpj @mpilgrem

There are continuous attempts to apply GfRC not only to HF itself, but also to other bodies. Currently most of them are void and null at best, because GfRC as written is not "retargetable". There are a few items which need to be fixed.

* As witnessed recently on more than one occasion, GfRC is not set in stone. An organisation subscribing to GfRC should be able to specify a version (say, v2025.05.09) instead of hotlinking an evolving text.
* Organisations should be able to copy a particular edition of GfRC to their own repository for future reference. This is currently illegal, because GfRC does not grant any license to subscribers. Should be one of Creative Commons licenses, I suppose.
* GfRC should say something like "the participating body" and "the leadership of the participating body" instead of "members of the Haskell Foundation" and "the chair of the Foundation, or the vice-chair or co-chair". I'm not sure what's the standard English legal wording for this, maybe "a group refering to these Guidelines" and "leadership of the group refering to these Guidelines" are most suitable.
* It should be made clear that only the section under "Guidelines For Respectful Communication" is normative and the rest ("Preamble", "Motivation", "Acknowledgements") are not. E. g., an organisation may seek to impose GfRC on its employees, a discussion forum may impose GfRC on all participants, but both cases would contradict Preamble. Similarly, motivations may vary, including "...because my employer told me so" or "...because it's a precondition for affilication with HF" and such.

Das Issue nennt keine Datei, keinen Test und keinen Einstiegspunkt. Beginne damit, das GfRC-Dokument zu finden und die Diskussion mit sieben Kommentaren zu prüfen, und kläre die Fragen zur Versionierung, Lizenzierung, Retargeting-Sprache und zum normativen Geltungsbereich mit den Maintainer:innen, bevor du Änderungen vornimmst; als erledigt gilt die Aufgabe, wenn ein abgestimmter, wiederverwendbarer Leitlinientext vorliegt.

Vom Indexierungsmodell aus dem Issue-Text verfasst.