docs: add capacitive touch element tutorial with polygon smtpads

[victorjzq]

/claim tscircuit/tscircuit#786

Adds a tutorial covering how to create capacitive touch elements using polygon SMT pads with solder mask in tscircuit.

This is the docs tutorial checklist item from tscircuit/tscircuit#786:

PR to tscircuit/docs with how tutorial describing how to create a capacitive touch element

What this tutorial covers

• How capacitive touch sensing works on PCBs (pad → solder mask → capacitance change)
• Using shape="polygon" on <smtpad> with coveredWithSolderMask={true} — the core feature from #786
• Single diamond-shaped touch button with interactive PCB preview
• Multi-pad touch surface (4 pads) with configurable spacing
• Custom polygon shapes (octagon approximation for circular pads)
• Wiring pads to a touch controller IC via named nets
• Design tips: pad sizing, solder mask thickness, spacing, ground plane, trace routing

Differentiation from PR #517

PR #517 (building-a-capacitive-touch-slider) covers linear sliders with rect shaped pads. This tutorial focuses on:

• Polygon-shaped pads — the specific circuit-json feature highlighted in #786
• Touch buttons/elements (not sliders) — as the bounty item title specifies "capacitive touch element"
• Complete end-to-end design tips and touch controller wiring

Closes part of tscircuit/tscircuit#786

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Project	Deployment	Actions	Updated (UTC)
docs	Error		Mar 19, 2026 2:53pm