Add selection of Manual version

doc/utils/sphinx-config/conf.py.in

@@ -99,6 +99,16 @@ def get_git_info():
         pass
     return git_n_date
 
+def get_git_commit():
+    import subprocess,time
+    try:
+        commit = subprocess.run(['git','rev-parse','HEAD'],stdout=subprocess.PIPE,stderr=subprocess.PIPE)
+        if commit.returncode == 0:
+            return commit.stdout.decode()
+    except:
+        pass
+    return ''
+
 # The version info for the project you're documenting, acts as replacement for
 # |version| and |release|, also used in various other places throughout the
 # built documents.
@@ -108,6 +118,8 @@ version = get_lammps_version()
 # The full version, including alpha/beta/rc tags.
 release = get_git_info()
 
+git_commit = get_git_commit()
+
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
 #language = None
@@ -389,3 +401,18 @@ breathe_domain_by_extension = { 'h'   : 'cpp',
                                 'cpp' : 'cpp',
                                 'c'   : 'c',
                                 }
+
+# allows showing multiple versions
+try:
+   html_context
+except NameError:
+   html_context = dict()
+
+html_context['display_manual_versions'] = True
+html_context['current_version'] = version
+html_context['git_commit'] = git_commit
+html_context['versions'] = [
+  ('latest', 'https://docs.lammps.org/'),
+  ('8 Apr 2021', 'https://lammps.sandia.gov/doc/')
+]
+html_context['downloads'] = [('PDF', 'Manual.pdf')]