[gatsby-theme-ocular] Works with node v12, but not node v14

[chrisgervang]

On node v14 gatsby markdown is not compiling successfully, reported in visgl/hubble.gl#179

A fresh node 12.22 and repo install fixed gatsby, but it's not yet known why or how to get node 14 compatibility.

vis.gl repos runs tests on node 14, so it would be ideal to write docs on node 14 too.

[ibgreen]

Not sure if is relevant but gatsby-theme-ocular is still on gatsby v2. It may be that Node 14 support requires moving to gatsby v3?