docs: drop options.md from the repository

Generate the file on the readthedocs builder using Nix. Since there is
no root access or user namespaces, we have to use proot (see
https://nixos.wiki/wiki/Nix_Installation_Guide#PRoot).

.readthedocs.yaml

@@ -8,6 +8,14 @@ build:
   os: ubuntu-22.04
   tools:
     python: "3"
+  apt_packages:
+    - nix
+    - 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"
 
 sphinx:
    configuration: docs/conf.py