[Sync-EN] Revise the Yaconf extension documentation

[lacatoire]

Syncs the reference/yaconf/ pages with php/doc-en#5801.

• book.xml: the introduction is rewritten around what the extension actually does: parsing at startup with no per-request I/O, zero-copy retrieval, the 1.2.0 compacted storage block, the copy-on-write sharing between PHP-FPM workers, the ZTS restriction on automatic reloading, the 1.2.0 recursive sub-directory loading, and the attack-surface argument for keeping the INI files outside the web tree.
• setup.xml: documents the three installation routes, PECL, PIE (pie install laruence/yaconf, since 1.2.0) and building from source, each with its own example.
• ini.xml: yaconf.directory comes first, with its real default (empty, not /tmp/conf/), the .ini extension requirement and the recursive sub-directory behaviour, plus the INI syntax and section inheritance examples moved here from the book. yaconf.check_delay explains that 0 means checking on every request, not that a restart is required, and gains the note that the directive only exists in non-ZTS builds.
• Yaconf::get() and Yaconf::has(): the dot notation is documented, including the 64 nesting levels and the sub-directory addressing, with worked examples; Yaconf::get() documents the default parameter and what is returned when the key is missing.
• Adds Yaconf::__debug_info(), translating the new page, with the warning that it exists only for the extension's own test suite.

reference/yaconf/versions.xml is not added: it is EN-only infrastructure with no Russian counterpart.

EN-Revision bumped to 3b052562d228be18fa6dce221df7e375469fb7ad on the six files.

Fixes: #1702