[WIP] docusaurus upgrade

[captbaritone]

Current failures can be found here: https://gist.github.com/captbaritone/654c78e85f7f22450a67dfeee645431b

[captbaritone]

Motivation for attempting this upgrade is to see if facebook/docusaurus@e346149 will fix the issue reported in #5039 where links of the form ./path/to/file.md/#foo (note the trailing slash) are not reported as a build time error enough though the links are broken.