docs: Improve documentation for database objects

[bvanelli]

Improves documentation on many fields, including:

• Explain that bold transactions cannot be achieved via this API
• Add a docstring to most useful database tables
• Document table methods so that they appear on the documentation
• Add one basic example using relationships and properties, using the budget as an example

Closes #105
Closes #107

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.90%. Comparing base (b3df262) to head (71ae5fa).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #109   +/-   ##
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  Coverage   97.90%   97.90%           
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  Files          16       16           
  Lines        2295     2295           
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  Hits         2247     2247           
  Misses         48       48           

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