Fix multiple broken links on the trailblazer site

[michaellennox]

This commit fixes multiple broken links on the trailblazer site.

In the process I also discovered the following broken links but was unsure where you might want them pointed or if they should be deleted:

• Link to pipeline in representable/3.0/function_api
• Link to hire you in representable/3.0/xml
• Link to gemgem to demonstrate auto-including Model::ActiveModel for all CRUD operations in operation/api

Hope this is helpful, thanks for all the work on trailblazer.