[Docs][V1] Prefix caching design (#12598)

- Create v1 design document section in docs.
- Add prefix caching design doc.

@WoosukKwon @ywang96

---------

Signed-off-by: Cody Yu <hao.yu.cody@gmail.com>

docs/source/index.md

@@ -153,6 +153,13 @@ design/automatic_prefix_caching
 design/multiprocessing
 :::
 
+:::{toctree}
+:caption: V1 Design Documents
+:maxdepth: 2
+
+design/v1/prefix_caching
+:::
+
 % How to contribute to the vLLM project
 
 :::{toctree}