Resources: handle intermediate pages

[veksen]

Currently, the sub directories (from the breadcrumbs) will lead to a 404.

Two big things (maybe separate issues?):

• Languages should/could have a dedicated page with some intro, links, and then a tree of the articles (reuse the algo from the https://tph.netlify.com/resources page). This page needs to be created in gatsby-node
• Sub-directories.... either we generate pages for them (via gatsby-node), or we stop linking to them all-together in the breadcrumbs. Maybe there isn't a huge benefit to sub-directories, so probably just not linking to them would make more sense...

[veksen]

fixed the dedicated language pages in #196, moved the rest into a standalone issue