update user_guide.md w/versions (#1049)

Co-authored-by: WenMeng-NOAA <48260754+WenMeng-NOAA@users.noreply.github.com>

doc/user_guide.md

@@ -2,6 +2,15 @@
 
 # Unified Post-Processing (UPP)
 
+The UPP code is publicly available on GitHub at https://github.com/NOAA-EMC/UPP.
+
+## Documentation for Previous Versions of UPP
+
+* [UPP Version UPP-SRW-v2.2.0](upp-srw-v2.2.0/index.html)
+* [UPP Version 11.0.0](upp_v11.0.0/index.html)
+
+## Background Information
+
 The Unified Post Processor (UPP) software package is a software
 package designed to generate useful products from raw model
 output. The UPP is currently used in operations with the Global
@@ -33,9 +42,10 @@ techniques. Examples of UPP products include:
 - Radar reflectivity products
 - Satellite look-alike products
 
-**Prerequisites**
+## Prerequisites
 The UPP requires certain NCEPLIBS packages to be installed via the spack-stack project. For instructions on installing these packages as a bundle via spack-stack, see: https://spack-stack.readthedocs.io/en/latest/. The UPP/modulefiles directory indicates which package versions are used and supported on Level 1 systems.
 
+## Community Support
 Community support for the Unified Forecast System (UFS) UPP in FV3-based applications is provided by the
 Earth Prediction Innovation Center (EPIC). Community support for the UPP with WRF is no longer available. 
 
@@ -47,4 +57,3 @@ Earth Prediction Innovation Center (EPIC). Community support for the UPP with WR
 
 Code Managers: Wen Meng (EMC), Huiya Chuang (EMC), Fernando Andrade-Maldonado (EPIC)
 
-