docs(readme): fix the Triple Bonnet mapping, install prerequisites and backup names

- README: the Adafruit Triple Bonnet uses `regular` (3 outputs), not
  `regular-pi1` (1 output) -- src/matrix_support.py MAPPING_OUTPUTS, and
  the README's own hardware_mapping section; the template default mapping
  is adafruit-hat, the PWM mod switches it to adafruit-hat-pwm; manual
  install only needs git up front (first_time_install.sh installs
  python-dev-is-python3, cmake, ninja-build etc.; cython3/scons are not
  used); the Pi Zero 2 W is a supported low-memory board, consistent with
  PRODUCT.md, LOW_MEMORY_BOARDS.md and the installer's low-memory build;
  fix the "First_time_install.sh" spelling, an orphan "2." list item and
  the hello-world starter link (it lives in the plugins monorepo).
- CONFIG_DEBUGGING: automatic backups are
  config/backups/config.json.backup.<YYYYMMDD_HHMMSS_ffffff> (five kept),
  not config_YYYYMMDD_HHMMSS.json.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

docs/CONFIG_DEBUGGING.md

@@ -292,9 +292,12 @@ cp config/config.json config/config.backup.json
 
 ### Automatic Backups
 
-LEDMatrix creates backups before saves:
+LEDMatrix creates backups before saves (`src/config_manager_atomic.py`):
 - Location: `config/backups/`
-- Format: `config_YYYYMMDD_HHMMSS.json`
+- Format: `config.json.backup.YYYYMMDD_HHMMSS_ffffff` (microseconds last),
+  plus a matching `config_secrets.json.backup.<timestamp>` when a secrets
+  file exists
+- The five most recent are kept
 
 ### Recovery
 
@@ -303,7 +306,7 @@ LEDMatrix creates backups before saves:
 ls -la config/backups/
 
 # Restore from backup
-cp config/backups/config_20240115_120000.json config/config.json
+cp config/backups/config.json.backup.20240115_120000_000000 config/config.json
 ```
 
 ## Troubleshooting Checklist