Make markdown link handling less fragile

[vkleen]

In #445 I implemented a hacky fix for an issue we noticed when addressing #335. Namely, some links on the user manual markdown pages would first throw a 404 error but then redirect to the correct page anyway.

The going hypothesis is that we're using the gatsby redirect handling wrong in some subtle way. But in the interest of time, I made a somewhat fragile gatsby-transformer-remark plugin that rewrites links matching a regular expression ^\./(.*)\.md$ to /user-manual/\1. In particular, it makes strong assumptions on the topology of the website.

This issue is intended to track progress on this front and to remind us of our sins.