docs: add model docstrings and inline pricing metadata mixin

[vdusek]

Split out from #950 (the other half is the to_camel alias refactor).

Adds concise class and field docstrings to all @docs_group-decorated models that were missing them:

• events/_types.py — SystemInfoEventData (8 resource-usage metric fields) and the 8 event wrappers (name / data).
• _charging.py — the pricing-info models, plus module-level constant docstrings and a few inline comments converted to docstrings.

Also inlines the private _RelaxedPricingMetadata mixin into the four pricing subclasses so each documents its own relaxed fields. This is behavior-preserving (field order, aliases, and requiredness are unchanged).

This PR keeps the manual field aliases and branches from master independently of #950. Once #950 merges, this branch will need a rebase to resolve the overlapping changes in _charging.py / events/_types.py.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.64%. Comparing base (10203bc) to head (ed80953).
⚠️ Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #956      +/-   ##
==========================================
+ Coverage   86.38%   86.64%   +0.26%     
==========================================
  Files          48       48              
  Lines        2916     2966      +50     
==========================================
+ Hits         2519     2570      +51     
+ Misses        397      396       -1     

Flag	Coverage Δ
e2e	?
integration	58.56% <100.00%> (+0.74%)	⬆️
unit	74.98% <100.00%> (+0.42%)	⬆️

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