Make any web access to an unversioned doc (next) also available via "latest"

[mmattel]

For consistency reasons, we should make all unversioned documentations like docs, branding, web and ocis also available with latest.

Examples:
https://doc.owncloud.com/docs/latest/* --> https://doc.owncloud.com/docs/next/*
https://doc.owncloud.com/branding/latest/* --> https://doc.owncloud.com/branding/next/*

This would then be inline with versioned documentations like, whre the version is handled automagically:
https://doc.owncloud.com/server/latest/* --> https://doc.owncloud.com/server/<actual version>/*

Tasks:

• Make a request to sysadmins to implement the redirects on the webserver