Homogenize link style

[dchiquito]

It was perhaps premature to close #309. While testing lychee for use in the CI checker, I discovered a bunch of relative links that were not being generated correctly.

Whenever Hugo encountered a relative path that it couldn't resolve, it just ignored it. By default, lychee ignores relative paths since it doesn't know what they are relative to. This results in false negatives whenever Hugo can't resolve a relative path.

The solution is to run lychee against the /content source directory with --base ./content/. However, this means that all relative links need to refer to actual files. Hugo is smart enough to infer that /content/fundamentals/peers/ refers to /content/fundaments/peers.md, but lychee isn't.

The work required here is to fix up all relative links to explicitly refer to specific markdown files, and to fix any errors that uncovers.