doc organization (#1197)

* doc organization

* removing what is rccl file

* Update index.rst

[ROCm/rccl commit: 3301cdf59a]

projects/rccl/docs/sphinx/_toc.yml.in

@@ -1,12 +1,11 @@
 root: index
 subtrees:
-- entries:
-  - file: what-is-rccl
+
 - caption: API reference 
   entries:
-  - file: library-specification
+  - file: api-reference/library-specification
     title: Library specification
-  - file: api-library
+  - file: api-reference/api-library
   
 - caption: About
   entries: