Document right-to-left support

[mabruchet]

Adds a Front-Office page documenting right-to-left support, so an integrator building a custom theme inherits the rules rather than reverse-engineering them from Flexy.

What it covers

• The direction is deduced, never chosen — from the language code alone, against a list held in the core. Why there is no setting, no column and no migration, and what the list contains.
• Reading it — the lang_direction() Twig function and the lang_direction variable it supersedes, with the fallback spelled out, plus the PHP entry points.
• Writing styles that follow the reading direction — the split between a logical property and an explicit rtl: variant, and the note that a centring left: 50% is not a side.
• Directional icons — mirrored with a class on the wrapper, never with a second set of SVG files.
• Values that must not flip — prices, phone numbers, references, tracking numbers, with the two traps that cost real time: dir="ltr" on a block element re-resolves its own text-align, and merchant-typed text wants <bdi> rather than a forced direction.
• JavaScript widgets — read the attribute from the DOM, not a render variable, so they survive a re-render.
• Why there is no second stylesheet, since other platforms ship one and the question will come up.
• A checklist an integrator can run against their own theme.

What it says is out of scope

PDF invoices and delivery notes, transactional e-mails, translating a theme into a right-to-left language, adding such a language to the default install, and formatting numbers in local digits.

Part of Thelia issue #341. Describes what ships in thelia/thelia#3972, thelia-modules/TwigEngine#14, thelia-templates/flexy#218 and thelia-templates/default-twig#91 — worth merging once those are in.