I've never run Gollum, so will ignore that.
In Wikipedia (which of course is based on Mediawiki) you can link to a page from anywhere, but you do need to specify the namespace if you change namespaces. For instance within the English wikipedia:
[[convert]] -> a redirect to
https://en.wikipedia.org/wiki/Conversion
[[template:code]] ->
https://en.wikipedia.org/wiki/Template:Code
[[fr:convert]] ->
https://fr.wikipedia.org/wiki/Convert
Dokuwiki is similar, except that instead of a number of flat namespaces (such as Mediawiki), there is a hierarchy of namespaces.
Because of its hierarchical nature you have a number of option for linking between namespaces. Put simply (see the link Lars gave you for details) you need to navigate to the "other" namespaces. You can either specify it exactly
[[:appliances:blender]][/m] or relatively by going "up a level" and then coming down [m][[..:appliances:blender]].
Consider the point of a namespace, it is to isolate a set of names and allow duplication. You might want
recipies:hotpot[/m] for the dish from Lancashire and [m]appliances:hotpot[/m] for the cooking utensil. Namespaces allow you to do this natively, otherwise you need to name your pages [m]recipies-hotpot[/m] and [m]appliances-hotpot which is probably worse that using the namespace.
You can install a redirection plugin such as the
pageredirect[/m] plugin that allows you to have pages which silently jump to the appropriate page. In your case having a page [m]recipes:blender[/m] which was a redirect to [m]appliances:blender[/m] means that you only ever need to link to [m]blender and will be magically transferred.
HTH