[v4] Backend customizations introduction

[pwizla]

What does it do?

Adds a contextual introduction for Backend customization.
Will be used as a landing page (if needed for SEO/redirections).

Note: The various concepts (routes, controllers, policies, middlewares…), now split in separate files, will be updated in upcoming PRs.

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🔍 Inspect: https://vercel.com/strapijs/documentation/3WDGbAjasL1g2w8BQi2ocm77kuss
✅ Preview: https://documentation-git-dev-v4-backend-customizations-intro-strapijs.vercel.app

[kasonde]

Could we make this more explicit, i.e it receives requests that are forwarded to the controllers that handle them by the routes. or is that not necessary in this context?

Suggested change

	As Strapi runs an HTTP server, it receives requests, handled by [routes](#routes). These requests can come from API calls (e.g. via a client application) or from the Strapi's admin panel and plugins.
	As Strapi runs an HTTP server, it receives requests, handled by [routes](#routes). These requests can come from API calls (e.g. via a client application) or from the Strapi's admin panel and plugins.

[derrickmehaffy]

I wonder if we should expand this to include some of our custom objects here like ctx.request.route

[derrickmehaffy]

Suggested change

	Requests can be handled on any URL by adding [routes](/developer-docs/latest/development/backend-customization/routes.md) to Strapi. Using routes is the first step towards creating custom JSON APIs to be used later on in the client application.
	Requests can be handled on any URL by adding [routes](/developer-docs/latest/development/backend-customization/routes.md) to Strapi. Using routes is the first step towards creating custom REST APIs to be used later on in the client application.

[pwizla]

This sounded weird, indeed, but it was actually written like this ('JSON') by @alexandrebodin in the document he sent me. 🤔

[kasonde]

I figured it was okay in order to create the distinction, "JSON APIs" for both REST and GraphQL? 🤔

[derrickmehaffy]

We should probably specify there are global middlewares (ran on every request) or route middlewares (which are only ran when the specific route is called).

In v3 we only had global middlewares and v3 policies functioned as route middlewares but that's changed with the new policy system in v4

[pwizla]

Thank you for your very valuable feedback, everyone!

I'm closing this PR as, after discussing its content and structure with Mégane, we will do it another way:

• explanations for each section will be reused in the introduction of the dedicated files (routes, policies, controllers, etc.)
• I will create another PR with a shorter intro to backend customization that will act even more as a TOC page