Fix for latest_api references

The "latest_api" link in the top-level docs folder redirects
you to the Asterisk_21_Documentation section but doesn't change
the nav bar because "latest_api" doesn't actually appear in
the menu.  To make that work, the link has been renamed to
"Latest_API" and is now shown in the nav bar.

Author: gtjoseph

docs/.pages

@@ -7,6 +7,7 @@ nav:
   - Deployment
   - Operation
   - Development
+  - Latest_API
   - Asterisk_16_Documentation
   - Asterisk_18_Documentation
   - Asterisk_19_Documentation
@@ -15,4 +16,4 @@ nav:
   - Certified-Asterisk_18.9_Documentation
   - Test-Suite-Documentation
   - Historical-Documentation
-  
+