docs: create leading directories for redirects

This will be useful when removing entire directories from the docs,
which will happen in an upcoming patch.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7174>

docs/_exts/redirects.py

@@ -15,6 +15,7 @@ def create_redirects(app, docname):
         return
     for src, dst in redirects:
         path = os.path.join(app.outdir, '{0}.html'.format(src))
+        os.makedirs(os.path.dirname(path), exist_ok=True)
         with open(path, 'w') as f:
             f.write(create_redirect(dst))