docs: Add conditional js/python rendering (#5128)

# Overview

Adding conditional rendering logic to co-locate js and python documentation.

* `:::` conditional syntax can be used to switch between python only or js only content.
* Contains simple unit tests for `:::`
* PR adds set up for a way to implement a context switch between languages, but it will not be enabled until JS content is merged in.
* Contains a script that can add javascript documentation 

Implementation of: https://github.com/langchain-ai/langgraph/pull/5118

## Example

Example of conditional rendering / compilation.


```markdown

### Config (static context)

Config is for immutable data like user metadata or API keys. Use
when you have values that don't change mid-run.

Specify configuration using a key called **"configurable"** which is reserved
for this purpose:


:::python

This content will only be rendered for the python site.
:::

:::js
this content will only be rendered for the js / ts site.
:::

```

docs/_scripts/generate_llms_text.py

@@ -3,19 +3,21 @@
 import asyncio
 import glob
 import os
-from typing import TypedDict, List, Optional
-import pydantic
 import re
-from pydantic import BaseModel, Field
-from langchain_core.rate_limiters import InMemoryRateLimiter
+from typing import TypedDict, List, Optional
 
 import yaml
 from langchain.chat_models import init_chat_model
+from langchain_core.rate_limiters import InMemoryRateLimiter
 from mkdocs.structure.files import File
 from mkdocs.structure.pages import Page
+from pydantic import BaseModel, Field
 from yaml import SafeLoader
 
-from _scripts.notebook_hooks import _on_page_markdown_with_config
+from _scripts.notebook_hooks import (
+    _on_page_markdown_with_config,
+    _apply_conditional_rendering,
+)
 
 HERE = os.path.dirname(os.path.abspath(__file__))
 # Get source directory (parent of HERE / docs)
@@ -211,7 +213,9 @@ async def process_nav_items(nav_items: list[NavItem]) -> list[NavItem]:
     # Remove any items that start with http:// or https:// looking only for
     # local file at this stages.
     nav_items = [
-        item for item in nav_items if not item["url"].startswith(("http://", "https://"))
+        item
+        for item in nav_items
+        if not item["url"].startswith(("http://", "https://"))
     ]
     # Process items in parallel
     tasks = [process_single_item(item) for item in nav_items]