Document Locale::isRightToLeft (PHP 8.5)

[lacatoire]

This PR adds documentation for Locale::isRightToLeft(), which was introduced in PHP 8.5 but was not previously documented in the manual.

The new section describes the method behavior, its reliance on ICU for determining text direction, and provides a simple usage example.

Fixes #5013

[DanielEScherzer]

Methods are generally documented in their own dedicated files, that are then linked in the synopsis below (see the line <classsynopsisinfo role="comment">&Methods;</classsynopsisinfo> and below it)

Its been a decade, but the last new Locale method documented seems to have been 53e4cea and you can see how that was documented - just create a new file

Looking through the commit log, it seems the most recent method addition documentation was 053ae3f so that can also be used for comparison - in addition to creating the new file, the versions information gets updated

[lacatoire]

@DanielEScherzer The documentation has been moved into a dedicated isrighttoleft.xml file. It is now automatically picked up by the existing Methods; include in locale.xml, consistent with other Locale methods.

[Girgias]

Mainly markup nits.

[Girgias]

Minor nits, LGTM otherwise.