Add inter-class diagram generated by refscan to schema documentation

[eecavanna]

This is a more narrowly-focused spin-off from https://github.com/microbiomedata/nmdc-schema/issues/2188. It is the "class" counterpart to this "collection" ticket: https://github.com/microbiomedata/nmdc-schema/issues/2240

Here are the tasks that make up this ticket:

• [x] Add an inter-class diagram (dynamically generated based upon the current schema files)
• [x] Add a blurb to the visualizations.html explanatory web page that links to the diagram

Progress here is blocked by issue #2238.

[eecavanna]

Moving to next sprint. The implementation here is done. We are holding off on merging until https://github.com/microbiomedata/nmdc-schema/issues/2238 has been resolved.