docs: explain active scanning requirement for device discovery

[bluetoothbot]

What

Add a "Device discovery and active scanning" section to the README.

Why

"Some of my SwitchBot devices are discovered but not all" is a recurring
support theme (e.g. #299, #287). The cause is well understood by maintainers
but undocumented: SwitchBot puts the model identifier in the BLE service data,
which is only delivered in the scan response and therefore requires active
scanning. With a passive scanner the service_data is absent and detection
falls back to manufacturer data, which only resolves a handful of models.

How

Verified against the code: parse_advertisement_data resolves the model from
service_data first (_find_model_from_service_data / suffix); the
manufacturer-data fallback (_find_model_from_manufacturer_data) matches by
length only and covers few models. GetSwitchbotDevices().discover() already
uses an active scanner, so the section notes the gap surfaces when a passive
source feeds advertisements (e.g. a Home Assistant adapter or ESPHome proxy in
passive mode) and tells users to enable active scanning there.

Docs-only; no code changes.

Testing

No code changed. Confirmed prettier preserves the prose wrapping (no
proseWrap override; lines match the existing Troubleshooting section width).

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Autonomous PR by Kōan, an agent run by Nick (@bluetoothbot). A human is responsible for reviewing and merging.

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Quality Report

Changes: 1 file changed, 17 insertions(+)

Code scan: clean

Tests: passed (1300 passed)

Branch hygiene: clean

Generated by Kōan

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