Added internal links to docs with a helper script

[mattsmida]

The reference docs have been updated to include internal links to other documentation pages.

Closes #10.

[mattsmida]

This is a first step at updating the documentation with internal links. While adding the links, I found and manually updated or deleted links that were no longer relevant or not working. The next steps will be to make the same updates in the timewarrior repo directly, and then integrate the included script that adds markdown links into the update-docs github action.