Add ENV_VARS.md to mkdocs navigation and link from options page

Co-authored-by: pwwang <[email protected]>

Author: Copilot

docs/options.md

@@ -1,5 +1,7 @@
 Options are used to change some behaviors in `datar`.
 
+For environment variable configuration (such as controlling verb AST fallback behavior), see [Environment Variables](ENV_VARS.md).
+
 ## Available options
 
 ### allow_conflict_names

mkdocs.yml

@@ -57,6 +57,7 @@ nav:
     - 'Import datar': 'import.md'
     - 'Backends': 'backends.md'
     - 'Options': 'options.md'
+    - 'Environment Variables': 'ENV_VARS.md'
     - 'The f-expression': 'f.md'
     - 'Data': 'data.md'
     - 'Examples':