Docs: Add environment variables reference page

docs/sphinx/_toc.yml.in

@@ -5,7 +5,7 @@ subtrees:
   - file: what-is-rccl.rst
     title: What is RCCL?
 
-- caption: Install 
+- caption: Install
   entries:
   - file: install/installation
     title: Installation guide
@@ -14,7 +14,7 @@ subtrees:
   - file: install/building-installing
     title: Building and installing from source
 
-- caption: How to 
+- caption: How to
   entries:
   - file: how-to/using-rccl-tuner-plugin-api
     title: Using the RCCL Tuner plugin
@@ -31,12 +31,14 @@ subtrees:
   - url: https://github.com/ROCm/rccl/tree/develop/ext-net/example
     title: NCCL Net plugin examples
 
-- caption: API reference 
+- caption: API reference
   entries:
   - file: api-reference/library-specification
     title: Library specification
   - file: api-reference/api-library
-  
+  - file: api-reference/env-variables
+    title: Environment variables
+
 - caption: About
   entries:
   - file: license