Docs: `robots.txt` is not working in our main docs domain

[humitos]

If you go to https://docs.readthedocs.com/robots.txt, you get a 404

I suppose this is because we redirect https://docs.readthedocs.com/ to https://docs.readthedocs.com/platform/stable/ and there may be something happening with that particular redirect.

Another possibility is that because we use subprojects, the https://docs.readthedocs.com/robots.txt file belongs to the super project and we don't have that file there. However, if that would be the case, Read the Docs should return the default one.