website: file restructure

[jemgillam]

PostGraphile website needs some tidying up. Here's a handy hint:

Looks like you can persist the permalinks by putting something like:

permalink: :baseUrl/:docsUrl/:langPart/:versionPart/my/custom/link

in the frontmatter. Might be worth considering since the restructure is quite helpful for browsing the files.

More details: facebook/docusaurus#101 (comment)

Originally posted by @benjie in #466 (comment)