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/tests/unit_tests/test_conditional_rendering.py

@@ -0,0 +1,22 @@
+from _scripts.notebook_hooks import _apply_conditional_rendering
+
+
+CONDITIONAL_RENDERING = """
+above
+:::js
+js-content
+:::
+between
+:::python
+python-content
+:::
+below
+"""
+
+
+def test_conditional_rendering() -> None:
+    """Test logic for conditional rendering of content."""
+    output = _apply_conditional_rendering(CONDITIONAL_RENDERING, "js")
+    assert output.strip() == "above\njs-content\n\nbetween\n\nbelow"
+    output = _apply_conditional_rendering(CONDITIONAL_RENDERING, "python")
+    assert output.strip() == "above\n\nbetween\npython-content\n\nbelow"