Allow packages to provide a redirection map to preserve their URLs when refactoring their doc

[stof]

This is an idea coming from the Symfony documentation. To keep the flexibility of reorganizing the doc while avoiding to break existing links to the documentation everywhere on the web (which is bad for users), they came up with a way to define redirections in the doc. They simply define a redirection_map file at the root of the package, and the website uses it to redirect old URLs to the new location of the doc, keeping links working (the redirection map is actually for a given version of the doc of a package).

[everzet]

That's a cool idea. Longer down the road, though. Definitely after a first release.

[stof]

yeah, this is meant to be in a later milestone. But I wanted to open the issue to have a place to remember it.
We don't need it for the first release anyway. It is only needed once someone changes the URL of an existing documentation page, and we won't have any existing URL before the release.