chore(site): stop tracking generated site/ (improvise-1xfb.4)

site/ is web/assemble.sh's default output, and site/docs is
scripts/build-doc-site.mjs's. 27 generated pages under site/docs were
committed by accident and went stale. Nothing reads the tracked copy:
CI's build job assembles with `nix build .#site` into the store, the
harness assembles into harness/serve/, and build-web-release.sh
assembles into a temporary stage.

Remove site/ from the index, ignore /site/, and say so in
context/code/doc-site.md.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01VdfEGXA7aJ6ftMsCc9PJze

context/code/doc-site.md

@@ -181,6 +181,15 @@ The general form: when a check misses a defect, add the check that could have
 seen it rather than only fixing the defect, and ask what *question* the
 existing check was answering.
 
+**`site/` is build output and is not tracked** (improvise-1xfb.4). It is
+`web/assemble.sh`'s default `OUT_DIR`, and `site/docs` is this script's.
+Every reader builds its own copy from source: CI's `build` job runs
+`nix build .#site`, which assembles into the store; the harness assembles
+into `harness/serve/`; `scripts/build-web-release.sh` assembles into a
+temporary stage. For a while 27 generated pages under `site/docs` were
+committed by accident. They went stale, still saying 12 categories after the
+cap was raised, and nothing read them.
+
 ## 5. Why a node script and not a generator
 
 The landing page is hand-written HTML with no build step and one pinned CDN