Fix Version/s: None
Diagrams is code is useful for out docs-like-code because it means that diagrams can be expressed in plain text and only rendered on demand as part of the site build. No proprietary files from design software or binary exports need to be managed in the Git repository.
Probably the best diagrams-like-code solution right now is Mermaid, which GitHub also announced native support for. Mermaid can be included in Sphinx docs via sphinxcontrib-mermaid.
Mermaid provides support for flow charts, sequence diagrams, entity relationship diagrams, and more. It's likely superior to blockdiag and seqdiag.
One thing that Mermaid lacks is cloud architecture diagrams, which we typically use diagrams for. There is an extension for native support of diagrams, sphinx-diagrams that's in a "rough" state. What we might want to do is document how to add sphinx-diagrams to the extensions if needed, rather than bake in official support at this time. It'll be mostly just SQuaRE web service projects that use Diagrams.
See docs at https://documenteer.lsst.io/guides/diagrams.html