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[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 53.02%. Comparing base (feee96c) to head (5fdb171). :warning: 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     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[eleftherias]

@dmjb Do we still need this given the architecture docs here? https://github.com/stacklok/toolhive/tree/main/docs/arch