conf.py: Add support for loading localized class reference

It will look up a potential `classes/<lang>` folder in the parent repository
of https://github.com/godotengine/godot-docs-l10n and use it if provided.

This folder includes a pre-translated copy of the class reference XML, read
to be used by Sphinx. This turned out to be a simpler approach than generating
PO files for class reference strings in reStructured Text format, given that
their source format is XML and the conversion on a per `msgid` basis would be
quite tricky.

This approach is very hacky, as Sphinx doesn't seem to support any kind of
conditional statements in its `toctree` directives, so we have to outright
replace the content at build time. This should typically only be done on RTD
so it shouldn't end up polluting the Git history.

(cherry picked from commit a29b557361edab78662cf6e3450155f52b967276)

conf.py

@@ -264,6 +264,19 @@ def godot_get_image_filename_for_language(filename, env):
 
 sphinx.util.i18n.get_image_filename_for_language = godot_get_image_filename_for_language
 
+# Similar story for the localized class reference, it's out of tree and there doesn't
+# seem to be an easy way for us to tweak the toctree to take this into account.
+# So we're deleting the existing class reference and adding a symlink instead...
+if is_i18n and os.path.exists("../classes/" + language):
+    import shutil
+
+    if os.path.islink("classes"):  # Previously made symlink.
+        os.unlink("classes")
+    else:
+        shutil.rmtree("classes")
+
+    os.symlink("../classes/" + language, "classes")
+
 # Couldn't find a way to retrieve variables nor do advanced string
 # concat from reST, so had to hardcode this in the "epilog" added to
 # all pages. This is used in index.rst to display the Weblate badge.