feat(v6): add docs for astro/config module

[ArmandPhilippot]

Description (required)

• Adds docs for astro/config exports in our astro:config module reference:

  • defineConfig(): Not the easiest to describe... I was tempted by "Defines the configuration for" 😄
  • envField, I see two options (I chose the first one):
    1. We document everything here and we replace what's in Configuration reference with a link to that page. This declutters the configuration reference and we can more easily describe each envField methods.
    2. We only provide a link to the matching section in the Configuration reference.
  • fontProviders: pretty minimal because we already have a reference for all the providers
  • getViteConfig(): Added in Add getViteConfig helper astro#4154 (v1, so no Since). I don't know if there are other use cases than Vitest, linking to the Testing guide is probably enough.
  • mergeConfig(): moved from Programmatic Astro API with some rewording
  • passthroughImageService(): I reused the wording we had in the Images guide.
  • sessionDrivers: the object is pretty big, so I think it's fine to keep it minimal and use links, as with fontProviders.
  • sharpImageService(): not really added, but this is available from this module since fix: move service functions from astro/assets to astro/config so it can be imported astro#6995 and I'm not really quite sure about any use cases since Sharp is now the default image service.
  • validateConfig(): moved from Programmatic Astro API with some rewording

• Updates the env.schema configuration reference as a preview of what it might look like if we update the JSDoc (see envField). If we're happy with that, we'll need to make a PR in the core repo!

Things to consider:

• Should we add links to mergeConfig() and validateConfig() from the Programmatic Astro API page? I don't see any obvious place because the current content is focused on CLI commands executed programmatically. We might need to reword a bit the introduction if we want to add links.
• Are we okay with the configuration reference change? We can also keep the previous description and just add a link to envField.

Related issues & labels (optional)

• Suggested label: add new content, v6

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en/guides/environment-variables.mdx	Source changed, localizations will be marked as outdated.
en/reference/configuration-reference.mdx	Source changed, localizations will be marked as outdated.
en/reference/modules/astro-config.mdx	Source changed, localizations will be marked as outdated.
en/reference/programmatic-reference.mdx	Source changed, localizations will be marked as outdated.

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