docs: fix Read the Docs by using portable-nix

As of recently, Nix 2.6 from Ubuntu 22.04 became too old to evaluate
nixpkgs. A new-enough version of Nix is available as part of Ubuntu
24.04, but those newer versions of Nix aren't happy with our rather
primitive proot workaround anymore.

Thankfully, someone already made a version of Nix that does all the
heavy lifting for running in unprivileged environments like the one
Read the Docs provides. So we used that instead.

.readthedocs.yaml

@@ -5,17 +5,18 @@
 version: 2
 
 build:
-  os: ubuntu-22.04
+  os: ubuntu-24.04
   tools:
     python: "3"
   apt_packages:
-    - nix
+    - curl
     - proot
   jobs:
     pre_install:
-      - mkdir -p ~/.nix ~/.config/nix
-      - echo "experimental-features = nix-command flakes" > ~/.config/nix/nix.conf
-      - proot -b ~/.nix:/nix /bin/sh -c "nix build -L .#optionsDoc && cp -v result docs/options.md"
+      - curl -L https://github.com/DavHau/nix-portable/releases/latest/download/nix-portable-$(uname -m) > ./nix-portable
+      - chmod +x ./nix-portable
+      - ./nix-portable nix build --print-build-logs .#optionsDoc
+      - ./nix-portable nix store cat $(readlink result) > docs/options.md
 
 sphinx:
    configuration: docs/conf.py