allow cross assembly documentation linking

[Doraku]

not tested but if a referenced type is not present in the assembly, the generator will try to link to the dotnet api website. For types present in an other assembly which its documentation has been generated by DefaultDocumentation, there should be a way to link correctly to said documentation.

[Doraku]

a21de9f
ability to generate a links file with a simple format:

{base url to add to link}
{element id like in the documentation xml file (ex: T:System.Int32)} {url relative to previous base url}
...

Example

https://github.com/Doraku/DefaultEcs/tree/master/documentation/api
P:DefaultEcs.Entity.IsAlive DefaultEcs-Entity-IsAlive.md
M:DefaultEcs.Entity.IsEnabled DefaultEcs-Entity-IsEnabled().md
M:DefaultEcs.Entity.Enable DefaultEcs-Entity-Enable().md
M:DefaultEcs.Entity.Disable DefaultEcs-Entity-Disable().md
M:DefaultEcs.Entity.IsEnabled``1 DefaultEcs-Entity-IsEnabled-T-().md
M:DefaultEcs.Entity.Enable``1 DefaultEcs-Entity-Enable-T-().md

easy enough to create one by hand for needed links, can change the base url whenever needed in the same links file. Need more testing