[EPIC] Write and adapt developer documentation for v9

[MatShir]

We need to create the 9.x branch for the developer documentation, and populate it with the v9 added and changed content:

• Create a “Notable changes” section for 9.0, similar to https://devdocs.prestashop-project.org/8/modules/core-updates/8.1/
• Inside that section, document the framework & dependency changes between PS 8 and PS 9, especially Symfony, and how module developers need to adapt their code if they used these
  • Notable changes for PrestaShop 9 #1658
  • Notable changes for theme developers to document for v9 #1653
  • document all BC Breaks
  • Update the doc regarding the refacto of controllers

API documentation

• Create the builder documentation for the new Backoffice API: document how it is built (to help future developers who need to understand it), what endpoints are available in v9 build => Introduce Admin API chapter #1860
• Create the user documentation for the new Backoffice API: document how to use it => https://build.prestashop-project.org/news/2024/meet-prestashop9-api/

Symfony layout

• Create the builder documentation for the Symfony Layout: how it is built, major changes between legacy and Symfony layout, how module developers should adapt their code for v9 => https://devdocs.prestashop-project.org/9/modules/core-updates/9.0/#symfony-layout

To do: better describe each item, how to organize it, how to integrate it into devdocs

[kpodemski]

Requires a better understanding of the impact:

1. Twig component: Make Header independent PrestaShop#33775
2. Introduce ShopContext and EmployeeContext sub context services PrestaShop#33223 - before and after recommendations + improvements for the https://devdocs.prestashop-project.org/9/development/components/context/
3. Changes to translations may require even more details, especially regarding the usage of a new translation method, possibly adding a way to easily export .xlf for a module, etc. (Fix applying special char on translations PrestaShop#30415, Use htmlspecialchars on trans parameters and deprecate _raw parameter PrestaShop#31900)
4. Add to docs about ObjectModel: Fix untranslated field names on Attribute Group error messages PrestaShop#33624

To be continued.

[kpodemski]

Hooks to document:

1. actionBackOfficeLoginForm
2. actionEmployeeRequestPasswordResetForm
3. actionPresentCategory
4. actionPresentSupplier
5. actionPresentManufacturer
6. actionPresentStore

[kpodemski]

Remember about adding Carriers page to the list of migrated pages (behind the feature flag).

[kpodemski]

This page has to be adjusted because it mentions Guzzle.