Host doxygen documentation for multiple releases (ufs-community#644)

Update the user_guide.md file to list a table of contents for each release.

Fixes ufs-community#610.

docs/user_guide.md

@@ -8,6 +8,13 @@ Utilities for the NCEP models. This is part of the
 The UFS_UTILS code can be found here:
 https://github.com/ufs-community/UFS_UTILS.
 
+## Documentation for Previous Versions of UFS_UTILS
+
+* [UFS_UTILS Version 1.6.0](ver-1.6.0/index.html)
+* [UFS_UTILS Version 1.5.0](ver-1.5.0/index.html)
+* [UFS_UTILS Version 1.4.0](ver-1.4.0/index.html)
+* [UFS_UTILS Version 1.3.0](ver-1.3.0/index.html)
+
 ## The Utilities
 
 - <a href="chgres_cube/index.html">chgres_cube</a> - Creates cold