docs: add experimental advanced routing reference page

[matthewp]

Description (required)

Documents the upcoming experimental.advancedRouting flag.

For Astro version: 6.2. See astro PR #16366.

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en/reference/api-reference.mdx	Source changed, localizations will be marked as outdated.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

Warnings reference

Icon	Description
🔄️	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]

Thanks Matthew, the wording looks good to me! No suggestions to improve that, only for the formatting:

• We usually don't put the arguments in the headings (ie. middleware() instead of middleware(state, next)). This helps make the links easier to parse when sharing them: #middleware instead of #middlewarestate-next
• I think an API block for the methods is also helpful since some of them have different arguments or different return types.

I also noticed a few em dashes. We usually don't use them in docs. I don't know if we want to stay consistent with this or not. I don't have a strong opinion.

Question: Do we need a section for ContextProvider?

I see that the FetchState types often refer to create and finalize. Maybe it would be helpful to document it so we can link to ContextProvider.create() and ContextProvider.finalize(). I think this helps users navigate easily to between related sections.

[yanthomasdev]

Thanks @matthewp, just adding a few suggestions to remove all these emdashes that are not necessary, and then LGTM

[matthewp]

@ArmandPhilippot I've updated this PR to also document cookies.consume().

[ArmandPhilippot]

Thank you! This looks ready to me! 🙌🏽

[yanthomasdev]

LGTM!