Thorium BUILD instruction

THORIUM_BUILD.md

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+
+```
+git checkout master2
+
+# do some change
+
+git commit -a
+
+COMMIT_HASH=$(git rev-parse HEAD)
+
+git checkout --orphan build
+
+git rm -r  --cached .
+
+npm i && npm run build
+
+git add build/gh-pages/build build/gh-pages/web build/types -f
+
+cat << EOF > package.json
+{
+  "name": "pdf.js",
+  "type": "module",
+  "types": "build/types/src/pdf.d.ts",
+  "repository": {
+    "type": "git",
+    "url": "git://github.com/edrlab/pdf.js.git"
+  },
+  "engines": {
+    "node": ">=20"
+  },
+  "scripts": {
+  },
+  "license": "Apache-2.0"
+}
+EOF
+
+git add package.json
+
+git commit -m "build $COMMIT_HASH"
+
+git push origin build
+
+git checkout master2 -f
+
+git prune
+```