in URLs, git-main -> 21 or v21.0

hugo/hugo.toml

 # baseURL terminates with version number or "git-main"
-baseURL = "https://bornagainproject.org/git-main"
+baseURL = "https://bornagainproject.org/21"
 
 languageCode = "en-us"
 title = "BornAgain"
@@ -21,13 +21,13 @@ PygmentsStyle = "vs"
 
   release_number = "21.0"
   # version name is either major_release_number or "git-main" (must agree with baseURL)
-  version_name = "git-main"
+  version_name = "21"
 
   recommended_python_major = "3"
   recommended_python_minor = "11"
 
   # for links to source repository
-  url_blob = "https://jugit.fz-juelich.de/mlz/bornagain/-/blob/main"
+  url_blob = "https://jugit.fz-juelich.de/mlz/bornagain/-/blob/v21.0"
 
   # currently unused
   url_doxy = "https://bornagainproject.org/ext/api/git-main/user-API"