[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:

• [x] 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
  • [x] https://github.com/PrestaShop/docs/issues/1658
  • [ ] https://github.com/PrestaShop/docs/issues/1653
  • [x] document all BC Breaks
  • [ ] Update the doc regarding the refacto of controllers

API documentation

• [x] 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 => https://github.com/PrestaShop/docs/pull/1860
• [x] 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

• [x] 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