software: add a Hardware Layout field reference

[MUSTARDTIGERFPV]

The Web UI page warns that Hardware Layout is for advanced users, but never says what any field does. Total coverage on the site today is two sentences plus a warning, and hardware.json is not mentioned anywhere.

This adds software/hardware-layout.md, covering all 130 fields across the 17 sections of hardware-schema.js, with the key as it appears in hardware.json, the Web UI label, the type, and the description.

Generated, not hand written

The tables are generated from src/html/src/utils/hardware-schema.js by walking the file and reading title, id, label, type, desc and options. That means:

• The wording matches the tool exactly, so the page and the Web UI cannot drift in phrasing.
• Nothing is invented. Every description is the schema's own.
• It can be regenerated when the schema changes, rather than hand patched.

Two select fields, power_max and screen_type, carry no desc because their options are self describing, so their option labels are listed instead. power_pdet does have a description; it just uses double quotes because it contains an apostrophe, which an early pass of the extraction missed.

Platform restrictions come from the /* FEATURE: ... */ markers in the schema and are mapped to plain wording:

Marker	Rendered as
IS_TX	Transmitters only
NOT IS_TX	Receivers only
NOT IS_8285	ESP32 only
HAS_SX127X	SX127x radios only
NOT HAS_SX127X	Not on SX127x radios
NOT HAS_SX128X	Not on SX128x radios
HAS_LR1121	LR1121 radios only

The restriction is stated on the section, or on the individual field where it differs from its section.

Other content

The page explains what each type means, and that an empty or negative pin value means the pin is not fitted. That is how the firmware reads it, for example in getVoltageSourcesFromHardware, which treats null, '' and any negative number as absent.

Two warnings: a wrong value can stop the device booting or damage the hardware, and raising a power limit only changes what the device reports, not what the amplifier can deliver. The second is the existing warning from the Web UI page, kept because it is a real and common misunderstanding.

This also gives the fields the audit called out individually, screen_mirror, pwm_out_only, ant_group, vbat and vsrc1 to vsrc3, a documented home.

Notes for review

• I did not add a cross link from the Hardware Layout section of webui.md, to avoid conflicting with the Voltage Calibration hunk in the Web UI tabs PR. Worth adding once both land.
• The word list gains the technical terms the schema uses, mostly radio bus pins, power detection fields and voltage sensor names. Schmitt is added identically to the PWM receivers PR so the two merge cleanly.
• The page has no ranges, defaults or worked examples. Those are not in the schema and would have to be researched per field, so this is a complete reference of what the schema states rather than a complete tuning guide.