docs: preserve Bref php.ini scan directory

[KairiYasumatsu]

Summary

• document how to add a custom PHP configuration directory without replacing Bref's default scan directory
• add a serverless.yml example that keeps /opt/bref/etc/php/conf.d
• warn that setting only the custom directory prevents bref.ini from loading

Why

PHP_INI_SCAN_DIR replaces PHP's configured scan directory. The current example suggests using only a custom path, which can stop Bref's /opt/bref/etc/php/conf.d/bref.ini from loading and cause bundled extension configuration to be lost.

The explicit default path follows the working configuration reported in #487.

Validation

• npm run lint (0 errors; one existing no-explicit-any warning)
• npm run build in website (93 pages generated, including /docs/environment/php)
• verified the rendered HTML contains the configuration example and warning
• git diff --check

Fixes #487

AI assistance

AI assistance was used to research and prepare this documentation change. I reviewed the final diff and validation results.

[mnapoli]

beautiful, thanks!

it's still in draft, I'll wait till you mark it ready to merge.

[KairiYasumatsu]

Thanks. I've marked it as ready for review.