Mark experimental configuration settings programmatically

[fricklerhandwerk]

Problem

Descriptions of experimental settings need the information on "experimentality" baked into the documentation, but this is double book keeping.

Example:

nix/src/libstore/globals.hh

Lines 351 to 359 in d93c151

	> **Warning**
	> This is an experimental feature.
	To enable it, add the following to [`nix.conf`](#):
	```
	extra-experimental-features = cgroups
	use-cgroups = true
	```

Given we merge #7798, that information can be displayed automatically, and removed without maintainer intervention if the feature becomes stable. It would also allow more convenient control over presentation, ensuring it will be consistent.

Proposal

• merge Documentation: list experimental features in manual #7798
• adapt the rendering helper for settings and the code outputting the structured data it works on accordingly

Checklist

• checked latest Nix manual (source)
• checked open documentation issues and pull requests for possible duplicates

Priorities

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[Ericson2314]

Whoever does this, please feel free to talk to me!

• In aa663b7 (already merged) I added the basic infrastructure to track this info on settings
• Documentation: list experimental features in manual #7798 is now adapted so experimental features have xp-feature-${name} anchors, ready to be linked from elsewhere.

Between those too, this ought to be fairly straight-forward.