Feature request: Two way linking

a nice feature of Gatsby, [Roam](https://www.gatsbyjs.com/plugins/@aengusm/gatsby-theme-brain/), and [other knowledge tools](https://notes.andymatuschak.org/About_these_notes) is that it can tell you what pages link to the current page you are looking at. this helps people on old blogposts find newer content, or for people at api reference pages to find example tutorial pages, without having to go back and manually backlink everything.

I think contentlayer has all the knowledge required, but the indexing could be intensive. still maybe not a problem at typical scales