what-is-rccl (#1312)

* what-is-rccl

* create Installation instreuctions from README

* update README link

* Add using-nccl

* Add note about docs

* correct doc path

* sources to source

* correct docs link

docs/sphinx/_toc.yml.in

@@ -1,6 +1,14 @@
 root: index
 subtrees:
 
+- caption: Installation 
+  entries:
+  - file: install/installation
+
+- caption: How to 
+  entries:
+  - file: how-to/using-nccl
+
 - caption: API reference 
   entries:
   - file: api-reference/library-specification
@@ -11,4 +19,3 @@ subtrees:
   entries:
   - file: license
   - file: attributions
-