#1197 docs: --calibrate loads the model more than once, and starts it when …

Description

…it is done

Measuring the CPU worker count needs a fresh engine, so the calibration loads the model again, and when it is done it starts the model for normal use. A user who does not expect that sees the machine freeze three times with no explanation, and can read the healthy, idle server it leaves behind as a hang (I did, while checking this).

It now says so where the calibration is described (DETAILS.md) and where it is offered (INSTALL.md), which also says the server is already running when --calibrate returns, so it should not be started a second time.

Rebased onto the new `main` after the history cleanup. Replaces #1036.