docs: Remove dead links, consistent formatting...
docs: Remove dead links, consistent formatting...

This page was a real chore chore on the POC. In the event that a new doc site won't be approved by v20, this will at least clean up the broken links I found during my audit. I would like to eventually create this as a [data file](https://github.com/spf13/hugo/pull/2508#commitcomment-18705617) since md tables get unwieldy pretty quickly.
gohugoio/hugo,mdhender/hugo,fedelibre/hugo,gohugoio/hugo,fedelibre/hugo,digitalcraftsman/hugo,mdhender/hugo,gohugoio/hugo,mitchchn/hugo,mdhender/hugo,mdhender/hugo,anthonyfok/hugo,anthonyfok/hugo,fedelibre/hugo,anthonyfok/hugo,digitalcraftsman/hugo,mitchchn/hugo,fedelibre/hugo