Docs: add Power & Charging section to user guide (incl. LiPo safety)

[brickbots]

Summary

Adds a top-level Power & Charging section to the user manual (docs/source/user_guide.rst), placed between "How It Works" and "Adjusting Brightness". This addresses item #9 (Power & Charging FAQ) from the documentation enhancement priorities.

Subsections:

• The two USB-C ports — charge-and-power (rear) vs power-only (keypad-side, bypasses the switch), reusing the existing annotated power.jpeg, plus the slide-switch description.
• Charging — blue/green LED, ~3 hr from empty, charge with the switch off, and a note that the slow taper to green is normal.
• Battery life — 4–5 hr but activity-dependent, no on-screen gauge, abrupt shutoff, with a note cross-referencing power-save mode and the Settings Menu.
• Running on external power — ≥2A sources, the 10,000mAh ≈ a night rule of thumb, the battery-reserve trick, and a warning that 12V must go through a 5V step-down.
• Battery safety & care — the LiPo safety subsection: warnings to stop on swelling/heat/damage and not to puncture/open the cell, plus care habits (charge from the built-in port only, charge attended/away from heat, temperature range, storage, recycling) and a note that only the PiSugar S Plus is a compatible replacement.

Facts sourced from the support knowledge base's "Power & Charging Details"; safety guidance written in the manual's voice using the existing warning admonition style. No new images needed.

Test plan

• sphinx-build -b html -n source <out> builds successfully with no warnings referencing user_guide.rst; the new :ref: to the Quick Start's powering section resolves cleanly.
• Reviewed the rendered page in a local build.

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