You have to add entries to nixos/doc/manual/redirects.json, which look like this:

  "<identifier>": [
    "index.html#<identifier>",
  ]

Rationale and design documentation: https://github.com/NixOS/nixpkgs/blob/master/pkgs/by-name/ni/nixos-render-docs/README.md

Once the page-split is performed, you'll also have to consider the output path of the element referred to by <identifier>, then the shape will have to be

  "<identifier>": [
    "<path/to/output/location>.html#<identifier>",
  ]

In reply to @yut0:matrix.org
Do you know if there's a way to generate the html pages as well / if the equivalent exists for home-manager?

In reply to @yut0:matrix.org
Do you know if there's a way to generate the html pages as well / if the equivalent exists for home-manager?

In reply to @yut0:matrix.org
What's the canonical way of generating the options html pages? pkgs.nixpkgs-manual.override { nixpkgs = <your current instance>; } seems like the way to go for the main manual page, but it doesn't build the options page. pkgs.nixpkgs-manual.optionsDoc has derivations for asciidoc, markdown, and json, but no html.
Alternatively, it looks like you can directly call the same derivation hydra uses to build these pages, i.e. "${(import "${nixpkgs}/nixos/release.nix" { inherit nixpkgs; }).manualHTML.x86_64-linux}/share/doc/nixos", which does build all of the manual pages, but this seems cursed..