docs: testing guide, expanded overrides, updated config docs
- Added testing guide with 10 test suites covering install, uninstall,
config changes, override preservation, and key coverage
- Expanded admin overrides with 9 more Joomla Update component keys
- Added frontend offline page and error page override keys
- Updated configuration guide to v02.00.00 documenting brand_name,
company_name, support_url params and two-layer override system
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>
jmiller
f1d06bba99