Fix wrong 'changelog' and 'reference' links in docs

Both references were referencing links from Python because of our intersphinx
mapping in `conf.py`:

    intersphinx_mapping = {"python": ("https://docs.python.org/3", None)}

Because Python's docs explicitly define both references, Sphinx fallbacks to
them instead of generating implicit references as was expected.

Fix #6397

doc/en/changelog.rst

@@ -1,3 +1,5 @@
+.. _`changelog`:
+
 =========
 Changelog
 =========

doc/en/reference.rst

@@ -1,3 +1,5 @@
+.. _`reference`:
+
 API Reference
 =============