Docs: side menu to match the order (llms) (#557)

Small quick fix:

Suggest making the order of the menu the same as it is written on the
page (Getting Started -> Key Concepts). Before the menu order was not
the same as it was on the page. Not sure if this is the only place the
menu is affected.

Mismatch is found here:
https://langchain.readthedocs.io/en/latest/modules/llms.html

docs/modules/llms.rst

@@ -21,7 +21,7 @@ The following sections of documentation are provided:
    :name: LLMs
    :hidden:
    
-   ./llms/key_concepts.md
    ./llms/getting_started.ipynb
+   ./llms/key_concepts.md
    ./llms/how_to_guides.rst
    Reference<../reference/modules/llms.rst>