docs: document the real governor-smu config schema

[Weijtmans]

Closes #49.

The Configuration example on the governor page had drifted from what the smu branch actually ships, which is exactly the confusion in #49: the installed config.toml had keys and values the example lacked, and the page gave no way to tell which side to trust.

What changed:

• The example is now the shipped default-config.toml from the current smu branch, with every key annotated, and the page now says plainly: the installed file is the reference, trust it over any guide's example.
• Added the two keys the example was missing, with their semantics taken from the governor source: fix-freq under [gpu-usage] (overlays hwmon freq1_input with the true SMU-read clock, off by default) and [frequency-range] (initial operating range at service start; both it and runtime D-Bus range changes are clamped inside the span of the [[safe-points]] curve).
• Corrected the stale values: sample/adjust are 250/100_000 in the shipped default, [load-target] is 0.65/0.50. This answers the "my values differ from the example" half of the issue: the file was right, the example was old.
• The safe-points annotation now says what the curve is (the hard frequency limits plus the voltage per step) and that a reader's curve does not need to match anyone's example. The existing voltage-ceiling tip covers raising it past 2000 MHz and got one lead-in sentence to that effect.
• Replaced the bare "restart after changes" note with a warning that the config is read at service start only (there is no auto-reload; verified by editing the file and watching the running governor keep its old range) plus the busctl read of the live range as the authoritative check.

Direct answers to the four questions in #49: fix-freq is real, leave it false unless you want the frequency-reporting fix; [frequency-range] is real, it is the startup range clamp; keep your [load-target] values, the example was stale; your [[safe-points]] do not need to match the example, they are your board's curve.

mkdocs build --strict passes.