Extend advanced routing docs with new features added by the RFC

[matthewp]

Description (required)

Adds new docs for:

• fetchFile option
• extending Provider types
• the Fetchable. type.

Related issues & labels (optional)

Associated with withastro/astro#16723

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en/reference/experimental-flags/advanced-routing.mdx	Source changed, localizations will be marked as outdated.

Warnings reference

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🔄️	The source for this localization has been updated since the creation of this pull request, make sure all changes in the source have been applied.

[ArmandPhilippot]

Looks good to me overall but I left a few comments (section out of place, missing since, and a code snippet that looks a bit odd to me).

And, I see you have updated src/content/docs/en/reference/configuration-reference.mdx here but the content is different from packages/astro/src/types/public/config.ts. I think this is great to show the two examples but we'll need to apply the same changes in packages/astro/src/types/public/config.ts since this file is auto-generated.

[ArmandPhilippot]

Thanks, LGTM!

[yanthomasdev]

LGTM! Good work!