Make Page.Aliases more useful in multidimensional setups (note)

Make Page.Aliases more useful in multidimensional setups (note)

There are 2 fundamental changes related to alias handling in this PR:

1. the `Page.Aliases`` method returns a ready-to-use list of aliases, not the raw input data. This means that it's prefixed with correct dimension values (e.g. language code) and output format paths, and ready to use in e.g. Netlify's _redirects file.
2. We only render aliases for output formats that is both defined as `IsHTML` and `Permalinkable`.

See #14402