[IMP] website_sale: Google Merchant Center data source

To ensure the multi-channel promotion of products through various platforms (e.g., Google, Meta, Pinterest, TikTok), we need to provide data feeds so that our users can easily input their catalog into these platforms.

This commit adds the documentation on the new /gmc.xml file for Google Merchant Center. This XML file serves as a product feed to Google, ensuring synchronization between our platform and Google's services.

The product feed can be configured to utilize a pricelist and can be translated to target different locations. For instance, if the French language is installed and there exists a pricelist named “UE” with the currency EUR, then “/fr/gmc-UE.xml” represents a feed in French, where all prices are displayed in EUR and computed according to the UE pricelist.

See also: odoo/odoo#186976

task-4319930

content/applications/websites/ecommerce/products/catalog.rst

@@ -197,3 +197,104 @@ blocks are defined and highlighted on the page when **dragging-and-dropping** a
    **keywords** linked to the products or the eCommerce categories improves organic traffic.
    Additionally, each category has its own specific URL that can be pointed to and is indexed by
    search engines.
+
+Multi-channel promotion
+=======================
+
+Expand your reach by connecting your product catalog to platforms like **Google Merchant Center
+(GMC)**. This integration allows you to create an automated feed of your product catalog, making it
+easier to promote your products on Google Shopping and other platforms.
+
+By enabling the **Google Merchant Center Data Source**, your website will generate a dynamic
+`/gmc.xml` feed containing essential product information such as names, prices, descriptions,
+images, and availability. This feed can be customized to include multiple languages and pricelists,
+ensuring your products are displayed correctly for different regions and audiences.
+
+Here is an example of a product and the corresponding `/gmc.xml` generated file:
+
+.. image:: catalog/catalog-example-product.png
+   :alt: Example Product in Odoo
+
+The resulting XML output might look like this:
+
+.. code-block:: xml
+
+   <rss xmlns:g="http://base.google.com/ns/1.0" version="2.0">
+      <channel>
+         <title>Home | Your Website</title>
+         <link>https://yourwebsite.com/</link>
+         <description>This is the homepage of the website</description>
+         <item>
+            <g:id>FURN_6666</g:id>
+            <g:title>Acoustic Bloc Screens</g:title>
+            <g:description></g:description>
+            <g:link>https://yourwebsite.com/shop/acoustic-bloc-screens-23#attribute_values=4</g:link>
+            <g:image_link>https://yourwebsite.com/web/image/product.product/31/image_1920</g:image_link>
+            <g:availability>in_stock</g:availability>
+            <g:price>295.0 USD</g:price>
+            <g:sale_price>265.5 USD</g:sale_price>
+            <g:sale_price_effective_date>2024-12-01T00:00/2024-12-31T23:59</g:sale_price_effective_date>
+            <g:product_detail>
+               <g:attribute_name>Color</g:attribute_name>
+               <g:attribute_value>White</g:attribute_value>
+            </g:product_detail>
+            <g:item_group_id>23</g:item_group_id>
+            <g:product_type>Desks &gt; Components</g:product_type>
+            <g:shipping>
+               <g:country>BE</g:country>
+               <g:service>Standard delivery</g:service>
+               <g:price>14.99 USD</g:price>
+            </g:shipping>
+            <g:free_shipping_threshold>
+                  <g:country>BE</g:country>
+                  <g:price_threshold>1000.0 USD</g:price_threshold>
+            </g:free_shipping_threshold>
+         </item>
+      </channel>
+   </rss>
+
+Follow the steps below to enable and configure this feature.
+
+Configuration
+-------------
+
+Start by making the `/gmc.xml` feed accessible:
+
+- Go to :menuselection:`Settings --> Website`.
+- In the **SEO** section, enable the option **Google Merchant Center Data Source**.
+
+Next, set up the connection with Google Merchant Center:
+
+- Log in to your Google Merchant Center account.
+- Head over to the **Data sources** section by clicking on the gear icon :guilabel:`(⚙)` and
+  selecting **Data sources**.
+- Click on **Add product source**.
+- Add the URL of your product feed (`/gmc.xml`) as the link to your product source file.
+
+   Example: `https://yourwebsite.com/gmc.xml`.
+
+Localized feeds
+---------------
+
+Creating language-specific feeds for each country you sell in is essential. It ensures clarity and
+cultural relevance in product information, potentially boosting sales.
+
+To target mutliple languages and countries, append the desired language code to the URL. Example:
+Use `/fr/gmc.xml` for French.
+
+.. note::
+   The selected language must first be enabled in your website's settings.
+
+It is also possible to create different feeds for different currencies, which allows customers to
+view prices in their local currency.
+
+To enable this feature, create a pricelist with the foreign currecny and include it's name in the
+feed URL. Example: `/gmc-EUR.xml` for a pricelist named "EUR".
+
+.. note::
+   The pricelist must be selectable.
+
+.. seealso::
+   For a detailed explanation of all fields in the `/gmc.xml` file, refer to Google's documentation:
+   `Google Merchant Center Product Feed Specifications
+   <https://support.google.com/merchants/answer/7052112>`_.