Flatten Overview section: remove double Overview in sidebar

Replace how-it-works/index with its children (architecture,
oss-vs-saas) directly in the root toctree so they appear as
siblings of Welcome to Robusta instead of nested under a
duplicate Overview entry.

https://claude.ai/code/session_0184fwhrjC4ELrYQBMCbyhfS

Author: claude

docs/index.rst

@@ -6,7 +6,8 @@
    :hidden:
 
    self
-   how-it-works/index
+   how-it-works/architecture
+   how-it-works/oss-vs-saas
 
 .. toctree::
    :maxdepth: 4