Internal link gets replaced with `undefined` or requires odd pathing

[isaacbrodsky]

Please see uber/h3#274

There were two issues I had with a href links. The first is that when an internal link wasn't resolved, it was replaced with undefined. This silently broke the site, but really should have failed the build. I see the code for Markdown pages has changed in master so I'm not sure if this issue still exists.

The second issue was that for the page /docs/core-library/filters.md to reference /docs/core-library/h3Indexing.md, I need to write the link as ../h3indexing.md (note the .. and that the capitalization changed). I think this may be an issue with documents nested more than one level deep since I didn't see this on the React Vis site. I would have preferred either h3Indexing.md or /docs/core-library/h3Indexing.md.

[isaacbrodsky]

Is there a recommended way to add internal links in ocular-gatsby? @jckr