DOC: Document angle-of-attack drag inputs

[ting-hong-shieh]

Closes #767.

Pull request type

• ReadMe, Docs and GitHub updates

Checklist

• Docs have been reviewed and updated
• Ruff and Pylint pass locally
• CHANGELOG.md — no action needed; an LLM workflow auto-updates it after merge

Current behavior

RocketPy already evaluates power_on_drag_7d and power_off_drag_7d with angle of attack, sideslip angle, Mach number, Reynolds number and body angular rates. Flight supplies the current flight-state values, but the public Rocket constructor documentation still says drag callables must depend on Mach number only.

New behavior

The constructor documentation now describes the supported constant, Mach-only and seven-variable drag inputs. The rocket usage guide adds:

• the argument order and units for seven-variable callables;
• an angle-of-attack and Mach example;
• the header format for multivariable CSV tables; and
• the distinction between the backward-compatible Mach-only attributes and the full *_drag_7d functions.

This changes documentation only; the drag calculation and public API remain unchanged.

Breaking change

• No

Verification

• pytest tests/unit/rocket/test_rocket.py -q — 60 passed
• ruff check rocketpy/rocket/rocket.py
• pylint rocketpy/rocket/rocket.py — 10.00/10
• Sphinx warnings-as-errors build for the rocket usage page