refactor: Flatten triple-nested sections in API Reference for better discoverability

- Removed unnecessary intermediate section headers in Configuration, Data Models, Evaluation Framework, and CLI Reference
- Changed from triple nesting (Section > Subsection > Toctree) to double nesting (Section > Toctree)
- Fixed typo: 'canFeature Specifications' → 'Feature Specifications'
- Improved navigation: Users can now see all reference topics directly in the main index
- Enhanced discoverability: Fewer clicks to reach specific documentation pages

Author: josh-hhai

docs/reference/index.rst

@@ -88,9 +88,6 @@ Decorators & Functions
 Configuration
 ~~~~~~~~~~~~~
 
-Environment Variables
-~~~~~~~~~~~~~~~~~~~~~
-
 .. toctree::
    :maxdepth: 1
 
@@ -101,9 +98,6 @@ Environment Variables
 Data Models
 ~~~~~~~~~~~~
 
-Event and Span Formats
-~~~~~~~~~~~~~~~~~~~~~~
-
 .. toctree::
    :maxdepth: 1
 
@@ -122,9 +116,6 @@ Evaluation Framework
 Command Line Interface
 ~~~~~~~~~~~~~~~~~~~~~~
 
-CLI Reference
-~~~~~~~~~~~~~
-
 .. toctree::
    :maxdepth: 1