Redirect old doc pages to new locations

As part of the docs restructuring #8443 some pages are moved, so the old URLs now return 404. Some of these pages have a lot of incoming links so I think it’s important to keep them working.

First we need to create a list of pages we want to keep working (TODO).

Then we need to implement the redirects. I think there are two ways to go about that:

1. HTTP redirects - requires server configuration. Looks like ReadTheDocs can do it.
2. HTML redirects - requires placeholder pages with <meta refresh>. Can use a sphinx plugin to do it or implement one ourselves.

I think server redirects are easier and more recommended. I can add them unless someone objects (I would need permissions for RTD though).

[nicoddemus]

I like the stub approach, and I suppose it could even stay around “forever” given how much those links exist over the web. 👍

[The-Compiler]

I went ahead and added a RTD redirect from /en/latest/warnings.html -> /en/latest/how-to/capture-warnings.html, so at least that one works correctly. There’s probably more, and I have no idea how to handle this for multiple versions (there’s a “page redirect”, but doing that redirect for the old versions would be wrong…).