docs: document the exclude codegraph.json option (#999) (#1010)

#1009 added `exclude` (keep git-tracked dirs out of the index) but didn't
document it. Add an "Excluding a tracked directory" section to the site config
page (parallel to includeIgnored) and a brief note + example to the README,
covering the committed-theme/SDK case .gitignore can't handle.

README.md

@@ -606,6 +606,18 @@ add a negation — `!vendor/`. The defaults apply uniformly, so committing a
 dependency or build directory doesn't force it into the graph; the `.gitignore`
 negation is the explicit opt-in.
 
+`.gitignore` can't drop a directory you've **committed**, though. For a vendored
+theme or SDK that's checked into the repo (e.g. a Metronic theme under
+`static/`), list it under `exclude` in `codegraph.json` — gitignore-style
+patterns, matched against repo-root-relative paths, honored on index, sync, and
+watch:
+
+```json
+{
+  "exclude": ["static/", "**/vendor/**"]
+}
+```
+
 ### Custom file extensions
 
 If your project uses a non-standard extension for a [supported