docs(examples): use the instrument context manager in examples

[nhschwab]

Closes #51.

What

Migrate the example scripts from manual open() + try/finally/close() to the with form. Every category inherits the context-manager protocol from Instrument (open on entry, close on exit including on exception), so the with block is shorter, exception-safe, and showcases the idiomatic API.

The mirrored example pages under docs/guides/instrumentation/examples/ are auto-generated from the scripts (just gen-examples); regenerated in the same commit.

Migrated (24 scripts)

All examples/daq/*.py, dmm/dmm_basic.py, eload/eload.py, i2c/i2c_basic.py, i2c/i2c_command.py, modbus/minimal_example.py, modbus/labjack_t4_loopback_test.py, psu/psu.py, psu/psu_background_daemon.py, all publishers/*.py, test_rack_example/test_rack.py.

Scope notes (reconciled against recent commits)

• psu_background_worker.py (named in the issue) is now psu_background_daemon.py — migrated.
• examples/custom/simple_temp_controller.py already uses with — no change.
• examples/modbus/modbus_example.py and programmatic_config_example.py use ModbusDevice(..., autostart=True), which opens + starts polling in __init__. They never use the manual pattern, so wrapping them in with would re-open an already-open device. Out of scope, left as-is.
• Prose HAL guides (psu.mdx, daq.mdx, etc.) were intentionally not mass-converted: quickstart.mdx deliberately documents both explicit open()/close() and the with block as first-class, and many .open()/.close() occurrences there are transport-level (visa.open()) or driver method definitions, not instrument-lifecycle usage.

Behavior preserved where finally did more than close

• test_rack.py: the safe-state shutdown (disable outputs + settle) is kept in an inner try/finally inside the with block, so it still runs before the instruments close.
• daq_sine_wave_dac0_t4.py: the KeyboardInterrupt handler that parks the DAC at 0 V is kept as an inner try/except inside the with block.

Verification

just check passes (ruff format, mypy, ruff lint all clean); all example scripts byte-compile.

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