Move towards single-versioned documentation

[elstoc]

Given the problems we're having keeping the documentation up-to-date for each new version of the application, I propose that from here on we publish a single version of the documentation only, taken from the current master branch of this repository, and archive previous versions (so that they are still available but never rebuilt).

The documentation is generally published with a url schema like https://docs.darktable.org/usermanual/<version>/<language>/, where <version> is either a real version number (currently 3.6, 3.8, 4.0, 4.2, 4.4, 4.6 or 4.8) or the word "development" (for current master). There is also a "stable" version url that currently redirects to v4.8.

I propose that we keep the existing versions at their current path (as static files that are never auto-built again). In place of the "development" version, we publish "latest", which will still be a build from current master. In the future, any request with an unknown version number (i.e. any version except one of the "real version" list above) will redirect to "latest".

[elstoc]

At some future point we will also need to re-enable a separate development build with a larger number of included languages than the "latest" build and find some way to automate this. For now, this change will allow help links to work from the current version without us having to build another unnecessary 5.0 branch