Add developer guide and move architecture documentation

[ohno]

Summary

• add a Developer Guide based on the contribution workflow used by Antique.jl
• document local setup, development flow, extension procedures, and release guidance
• move the Mermaid dependency diagram from the README to the final Architecture section
• add the guide to the Documenter navigation and enable Mermaid rendering
• simplify the README documentation section to direct links

Why

The README mixed user-facing project information with contributor workflow and architecture details, while the documentation had no dedicated developer page. This made the development guidance easy to miss and left the dependency diagram outside the documentation site.

Impact

Contributors now have one structured guide for setting up the repository, testing changes, building documentation, and extending operators or solvers. The README is shorter, and the architecture diagram is rendered in the documentation site.

Validation

• git diff --check
• julia --project=docs --startup-file=no -e 'using Pkg; Pkg.develop(PackageSpec(path=pwd())); Pkg.instantiate(); include("docs/make.jl")'
• verified all 11 documentation pages were generated and the Developer Guide contains the rendered Mermaid block

Close #20

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This pull request was developed with assistance from Codex using GPT-5.6 Sol with High reasoning effort.