feat: add copy page button functionality and fix llms-text output (#5419)

* feat: add copy page button functionality and fix llms-text output

- Add copy page button with CSS and JS implementation
- Implement copy page hooks for MkDocs integration
- Fix HTML filtering and DOM text reinterpreted as HTML issues
- Update llms-text target to generate docs/llm.txt instead of docs/llms-full.txt
- Add necessary styling and package.json dependencies

* fix missing button in preview

* remove the over-processing

* disable API reference

docs/package.json

@@ -9,7 +9,8 @@
     "@langchain/core": "^0.3.38",
     "@langchain/openai": "^0.4.2",
     "msgpack-lite": "^0.1.26",
-    "nock": "^14.0.1"
+    "nock": "^14.0.1",
+    "he": "^1.2.0"
   },
   "devDependencies": {
     "@tsconfig/recommended": "^1.0.8",