Clarify php.ini search order and restructure Windows registry details

[joshtrichards]

This revises the "configuration file" section of install/ini.xml to make the php.ini search logic clearer:

• Replace the flat unordered list with a numbered list reflecting the actual two-phase logic
• Move the php-SAPI.ini vs php.ini fallback into the search order where it logically belongs
• Extract Windows registry key details into a dedicated <sect2> to declutter the main flow
• Wrap the existing environment variable / syntax content in a new <sect2> to fix mixed sectioned/non-sectioned siblings
• Move the Apache root directory <note> into the CWD list item it relates to
• Add a brief mention of PHP_INI_SCAN_DIR / --with-config-file-scan-dir in the high-level flow
• Fix WOW6421Node / WOW6432Node typo and malformed brackets in registry key references

Related: php/php-src#21503