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

.gitignore

@@ -61,6 +61,10 @@ GSYMS
 __pycache__/
 *.pyc
 
+# The assembled site, written by web/assemble.sh (and the doc generator's
+# default OUT_DIR). Build output: CI assembles it from source (improvise-1xfb.4).
+/site/
+
 # The release web capture, written to the repo root by
 # scripts/build-web-release.sh (improvise-oujb).
 /improvise-web.tar.gz