Use the @Modules Twig namespace for mail layouts added from a module

[boo-code]

Questions	Answers
Branch?	9.x
Description?	Both pages that show a module supplying its own mail Layout build the path with _PS_MODULE_DIR_, i.e. an absolute filesystem path. MailTemplateTwigRenderer passes that value to Twig\Environment::render(), which resolves template names, so an absolute path raises "Unable to find template" - the "Could not find layout file" in the report. Measured with a loader configured like the renderer's: the absolute name fails while @Modules/... and a path relative to a registered Twig path both render. The two pages now use the @Modules namespace.
Fixed ticket?	Fixes PrestaShop/PrestaShop#30290
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Why the core issue needs no core change

The code half of #30290 was already fixed: PR #38304 ("Fix mail theme scanner when used on a module
folder", 47a434d1143b, base 9.0.x, first release tag 9.0.0, present on upstream/9.2.x) makes
FolderThemeScanner emit @Modules/<relative> instead of @MailThemes when it scans a module folder.

What remains is a module supplying its own Layout path, which is what the docs teach - and the docs
still teach the broken form. Two earlier PRs (#30295, #31987) were closed, the second with kpodemski
pointing at #38304.

Measured

MailTemplateTwigRenderer:118 calls $this->twig->render($layoutPath, ...), so the path is a Twig
template name, not a filesystem path. With a loader configured like the renderer's:

/tmp/tw/mails/layouts/order_conf.html.twig     LoaderError: Unable to find template
@Modules/mails/layouts/order_conf.html.twig    renders
mails/layouts/order_conf.html.twig             renders

The file existed and its directory was a registered Twig path - an absolute name still fails, which is
exactly the "Could not find layout file" in the reporter's screenshot.

Scope

bapman81 reported one page; two teach the broken form and both are fixed:

• modules/concepts/mail-templates/extend-a-layout-from-module.md:101
• modules/concepts/mail-templates/add-a-layout-from-module.md:123

Each now uses '@Modules/' . $this->name . '/mails/layouts/...' - $this->name rather than a hard-coded
folder, so the example stays correct whatever the module is called - plus a {{% notice tip %}} block
explaining that the path is a Twig template name. The notice shortcode matches the repo's existing style
({{% notice tip %}} is used 35 times under modules/).