Canonicalize redirected pages correctly

[QuLogic]

In 3.4.0, we added a redirect for /tutorials/introductory/customizing.html from /users/customizing.html. The latter file was previously deleted after 2.0.2. When 3.3.4 was latest, we said the canonical link was 2.0.2/users/customizing.html, but now that a file has returned in that location, the canonical link is to /stable/users/customizing.html, which is a only meta-refresh to the right file.

I have not committed these changes. We should decide on what to do here, but if those old files will stop pointing to 2.0.2, then they should point to the final location instead of a meta-refresh.

[jklymak]

I'd say again, its not really worth the extra hassle? Folks get where they want to eventually, and will ideally not be linking old pages.

Conversely, I guess we could check the contents of the html for the refresh, but...

[QuLogic]

Well, in this case, I'm not worried about people getting there, but about the effect on canonicalization for search engines and whether it breaks something due to not pointing at the final page.

[jklymak]

I don't think thats a problem. In general we only want the web search to reach the canonical stable/, and other links should be discouraged on the search engines anyway... Its possible we will get some sort of penalty on the search terms, so we should keep an eye on that, but I'm not too concerned so long as "customizing matplotlib" gives us https://matplotlib.org/stable/tutorials/introductory/customizing.html as a top-level search....

But again, I'm no expert on SEO, so if this is really a problem we could get fancier...

[QuLogic]

For now, I've committed the changes when adding 3.4.1 docs. If it is an SEO problem, then we can modify this a bit.

[QuLogic]

Let's leave this as is. At least the double-redirects get to the right place eventually.