Investigate why link isn't working for some pages and investigate a mass migration to using it

[daveverwer]

As mentioned in #759 (comment), the site should be using the link tag wherever possible. However, it's not working quite right for some pages right now. For example, from documentation/server/index.md:

[here]({% link /sswg/incubation-process.md %} "SSWG Incubation Process")

Should work, but gives this error:

  Liquid Exception: Could not find document '/sswg/incubation-process.md' in tag 'link'. Make sure the document exists and the path is correct. in documentation/server/index.md
             Error: Could not find document '/sswg/incubation-process.md' in tag 'link'. Make sure the document exists and the path is correct. 
             Error: Run jekyll build --trace for more information.

Once this is figured out, let's investigate what it would take to convert site links on the major pages over to using link and post_url. See the documentation here: https://jekyllrb.com/docs/liquid/tags/#links