feat(sidebar): use base when creating link

[pikax]

create a link based on the absolute url, this might cause problems if the children start with '/'

[kiaking]

Yeah, I think we should handle links starting with /, because I think it's easier for users to add config like,

{ text: 'Guide', link: '/guide/' }

Although, I think we could just enforce users to not add trailing slash on base url 😳