docs: add unmaintained section

[ojeytonwilliams]

This is for projects that are still interesting, but are somewhat neglected.

As discussed here: https://github.com/freeCodeCamp/devdocs/pull/1572#issuecomment-862861426

@MasterEnoc I'm fine with deleting it, if you'd rather, but I thought it would be nice to keep older projects around for a while.