docs: update Electrode Guide with usage notes and product photos #317

[catamgalvan]

Description

This PR rewrites the Electrode Guide page, replacing the list of "buy here" links with technical descriptions, usage instructions, and real product photos for each electrode, cable, and adapter.

Closes #317

Changes Made

• Added technical descriptions: Wrote detailed copy for each electrode type (Gold Cup, Dry EEG Comb, EMG/ECG Foam Solid Gel, EEG Polymer, EEG Snap Electrodes) covering material, signal type, and intended use case.
• Added Usage notes: Introduced a consistent "Usage" block under each electrode explaining how to apply it to the body and connect it to an OpenBCI board.
• Restructured the page: Split content into "Electrodes" and "Cables & Adapters" sections.
• Included images: Added real product photos with callouts for every electrode, cable, and adapter, plus a GIF demonstrating polymer electrode placement on the velcro strap.
• Updated intro: Clarified that boards support multiple biosignals (EEG, EMG, ECG) and previewed that the guide also covers cables and adapters.