Update architecture diagrams

[WardBrian]

Now that Github natively supports diagrams like flowcharts and since we may be doing some architectural changes soon in #1182 #1184, I thought it would be good to update our existing diagrams/prose a bit.

By embedding it directly it keeps it self-documenting, responds to the viewers dark/light mode setting, and the nodes are even clickable to take you to the files mentioned.

Preview here: https://github.com/WardBrian/stanc3/blob/update-architecture-docs/README.md

Submission Checklist

• [x] Run unit tests
• Documentation
  • [x] OR, no user-facing changes were made

Copyright and Licensing

By submitting this pull request, the copyright holder is agreeing to license the submitted work under the BSD 3-clause license (https://opensource.org/licenses/BSD-3-Clause)

[rok-cesnovar]

Now that Github natively supports diagrams like flowcharts

Wow nice, when was this added.

[WardBrian]

Earlier this year, somehow months before they finally added $\LaTeX$ rendering