Generate the nix.conf docs from the source code

This means we don't have two (divergent) sets of option descriptions
anymore.

doc/manual/generate-options.jq

@@ -0,0 +1,11 @@
+. | to_entries | sort_by(.key) | map(
+  "  - `" + .key + "`  \n"
+  + (.value.description | split("\n") | map("    " + . + "\n") | join("")) + "\n\n"
+  + "    **Default**: " + (
+      if .value.value == "" or .value.value == []
+      then "*empty*"
+      elif (.value.value | type) == "array"
+      then "`" + (.value.value | join(" ")) + "`"
+      else "`" + (.value.value | tostring) + "`" end)
+  + "\n\n"
+) | join("")