Replace Sphinx search with Pagefind

[KiaraGrouwstra]

Sphinx's stock client-side search stems and splits identifiers like system.nix into ubiquitous terms (system + nix) and stores no positional data, so phrase/identifier ranking is impossible: searching system.nix did not (highly) rank guides/recipes/dependency-management.html, which contains the exact string.

Pagefind indexes the rendered HTML post-build and does real phrase/proximity ranking. Searching system.nix now ranks that page first.

• pagefind.yml: scope indexing to the article body, exclude UI chrome.
• default.nix: add pkgs.pagefind to nativeBuildInputs (also exposes it in the dev shell via inputsFrom).
• Makefile: run pagefind --site build/html after sphinx-build in the html target for local/CI parity, guarded with a clear error if the binary is missing.
• source/_templates/search.html: replace the Sphinx search page with a Pagefind UI, dropping the now-unused Sphinx search scripts and pre-filling from ?q= so the sidebar search box still works.

Verifying:

• before: https://nix.dev/search.html?q=%22system.nix%22#
• running this PR locally: nix-shell --run 'make html' && python3 -m http.server -d build/html 8000
• after: http://127.0.0.1:8000/search.html?q=%22system.nix%22

Assisted-by: Claude:claude-opus-4-8

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/discussion-of-flakes-from-2025-nixos-community-survey-report/78853/40

[nixos-discourse]

This pull request has been mentioned on NixOS Discourse. There might be relevant details there:

https://discourse.nixos.org/t/discussion-of-flakes-from-2025-nixos-community-survey-report/78853/41

[hsjobeki]

I like the idea.

Tested this locally.

I think excerpts could be longer.

New

Current

I'd like to try some other queries still. But pagefind is a sane default for a fully static page imho.

[KiaraGrouwstra]

I think excerpts could be longer.

i bumped the snippets up to 30 words now

[hsjobeki]

@KiaraGrouwstra : Resolved the conflicts.
The pagefind styling now follows the main theme, so dark mode also works.

Most notably i changed the description of your original commit message.
It still reads a bit AI-generated but at least is more understandable and doesn't just re-state the file-list I can already get from the diff.

I tested the change locally. Using pagefind makes sense to me, it also aligns better with the upcoming docs.nixos.org