Add comprehensive SPI programmer best practices guide with safety warnings and modern recommendations

[Copilot]

Addressing Review Feedback on Programmer Recommendations

Changes Completed ✅

• [x] Remove BeagleBone Black from main recommendations (slow, outdated)
• [x] Remove mention of screwdriver firmware (not maintained)
• [x] Update comparison table to remove BeagleBone Black
• [x] Renumber CH347 from #5 to #4 in recommendations
• [x] Ensure focus is on modern, commonly available options (Tigard, CH341A rev 1.6+, Raspberry Pi Pico, CH347)
• [x] Update Advanced Features section to remove BeagleBone Black debugging
• [x] Validate changes don't break documentation structure
• [x] Add specific speed specification for CH347 programmer (~60 seconds for 16MB chip)

Summary

Successfully removed all references to outdated BeagleBone Black programmer and unmaintained screwdriver firmware. The guide now focuses exclusively on modern, actively maintained, and commonly available SPI programmers that are appropriate for current hardware. All recommended programmers now include complete performance specifications for consistent comparison.

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