[2.2.x] Used extlinks for Django's source code.

Backport of a68c029e22 from master.

docs/conf.py

@@ -43,12 +43,6 @@ extensions = [
     "sphinx.ext.viewcode",
 ]
 
-extlinks = {
-    'commit': ('https://github.com/django/django/commit/%s', ''),
-    'cve': ('https://nvd.nist.gov/view/vuln/detail?vulnId=%s', 'CVE-'),
-    'ticket': ('https://code.djangoproject.com/ticket/%s', '#'),
-}
-
 # Spelling check needs an additional module that is not installed by default.
 # Add it only if spelling check is requested so docs can be generated without it.
 if 'spelling' in sys.argv:
@@ -100,6 +94,14 @@ else:
 # The "development version" of Django
 django_next_version = '3.0'
 
+extlinks = {
+    'commit': ('https://github.com/django/django/commit/%s', ''),
+    'cve': ('https://nvd.nist.gov/view/vuln/detail?vulnId=%s', 'CVE-'),
+    # A file or directory. GitHub redirects from blob to tree if needed.
+    'source': ('https://github.com/django/django/blob/stable/' + version + '.x/%s', ''),
+    'ticket': ('https://code.djangoproject.com/ticket/%s', '#'),
+}
+
 # The language for content autogenerated by Sphinx. Refer to documentation
 # for a list of supported languages.
 # language = None