Create domain model documentation

[dmjb]

This is mostly generated by Claude, but begins the process of creating some documentation around ToolHive's domain model.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 53.02%. Comparing base (feee96c) to head (5fdb171).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2184      +/-   ##
==========================================
- Coverage   53.05%   53.02%   -0.04%     
==========================================
  Files         222      222              
  Lines       28868    28868              
==========================================
- Hits        15315    15306       -9     
- Misses      12414    12420       +6     
- Partials     1139     1142       +3     

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